Fix use of -fvisibility=hidden with Clang in external/libcdr, external/libqxp

At least on macOS x86-64 you get a warning

> [build LNK] Library/libwpftdrawlo.dylib
> ld: warning: direct access in function 'std::__1::__shared_ptr_pointer<librevenge::RVNGInputStream*, std::__1::shared_ptr<librevenge::RVNGInputStream>::__shared_ptr_default_delete<librevenge::RVNGInputStream, librevenge::RVNGInputStream>, std::__1::allocator<librevenge::RVNGInputStream> >::__get_deleter(std::type_info const&) const' from file 'workdir/UnpackedTarball/libzmf/src/lib/.libs/libzmf-0.0.a(ZMFDocument.o)' to global weak symbol 'typeinfo for std::__1::shared_ptr<librevenge::RVNGInputStream>::__shared_ptr_default_delete<librevenge::RVNGInputStream, librevenge::RVNGInputStream>' from file 'workdir/UnpackedTarball/libcdr/src/lib/.libs/libcdr-0.1.a(CDRDocument.o)' means the weak symbol cannot be overridden at runtime. This was likely caused by different translation units being compiled with different visibility settings.

The reason is that libzmf uses -fvisibility=hidden and generates

> $ nm -m workdir/UnpackedTarball/libzmf/src/lib/.libs/libzmf-0.0.a | grep __ZTINSt3__110shared_ptrIN10librevenge15RVNGInputStreamEE27__shared_ptr_default_deleteIS2_S2_EE
> 0000000000006dd8 (__DATA,__const) weak private external __ZTINSt3__110shared_ptrIN10librevenge15RVNGInputStreamEE27__shared_ptr_default_deleteIS2_S2_EE

while libcdr erroneously does not use -fvisibility=hidden and generates

> $ nm -m workdir/UnpackedTarball/libcdr/src/lib/.libs/libcdr-0.1.a | grep __ZTINSt3__110shared_ptrIN10librevenge15RVNGInputStreamEE27__shared_ptr_default_deleteIS2_S2_EE
> 00000000000072b8 (__DATA,__const) weak external __ZTINSt3__110shared_ptrIN10librevenge15RVNGInputStreamEE27__shared_ptr_default_deleteIS2_S2_EE

The reason for that error is as follows:
workdir/UnpackedTarball/libcdr/configure.ac uses, among other things, the result
of

  AX_GCC_FUNC_ATTRIBUTE([visibility])

when deciding whether to use -fvisibility=hidden.  And the old ("serial 5")
workdir/UnpackedTarball/libcdr/m4/ax_gcc_func_attribute.m4 decides "no" if its
test compilation generates any warning output.  But Clang on macOS generates

> conftest.cpp:34:56: warning: target does not support 'protected' visibility; using 'default' [-Wunsupported-visibility]
>                     int foo_pro( void ) __attribute__((visibility("protected")));
>                                                        ^

and lots of

> conftest.cpp:2:9: warning: macro is not used [-Wunused-macros]
> #define PACKAGE_NAME "libcdr"
>         ^

(because of -Wunused-macros set for Clang in
solenv/gbuild/platform/com_GCC_defs.mk).

Same issue with external/libqxp, which would cause

> [LNK] Library/libwpftdrawlo.dylib
> ld: warning: direct access in function 'std::__1::__shared_ptr_pointer<librevenge::RVNGInputStream*, std::__1::shared_ptr<librevenge::RVNGInputStream>::__shared_ptr_default_delete<librevenge::RVNGInputStream, librevenge::RVNGInputStream>, std::__1::allocator<librevenge::RVNGInputStream> >::__get_deleter(std::type_info const&) const' from file 'workdir/UnpackedTarball/libcdr/src/lib/.libs/libcdr-0.1.a(CDRDocument.o)' to global weak symbol 'typeinfo for std::__1::shared_ptr<librevenge::RVNGInputStream>::__shared_ptr_default_delete<librevenge::RVNGInputStream, librevenge::RVNGInputStream>' from file 'workdir/UnpackedTarball/libqxp/src/lib/.libs/libqxp-0.0.a(QXPMacFileParser.o)' means the weak symbol cannot be overridden at runtime. This was likely caused by different translation units being compiled with different visibility settings.

<http://git.savannah.gnu.org/gitweb/?p=autoconf-archive.git;a=commitdiff;
h=df0894ad1a8195df67a52108b931e07d708cec9a> "ax_gcc_func_attribute: Revise the
detection of unknown attributes", even though it was apparently meant to fix
something different, nicely fixes this Clang issue, making AX_GCC_FUNC_ATTRIBUTE
correctly detect support for visibility now.

When building with Clang on Linux, there is no -Wunsupported-visibility about
__attribute__((visibility("protected"))), but all the -Wunused-macros are
present as well, which caused all the AX_GCC_FUNC_ATTRIBUTE checks to be mis-
detected as "no" there, too.

There are more uses of AX_GCC_FUNC_ATTRIBUTE in
workdir/UnpackedTarball/libcdr/configure.ac and
workdir/UnpackedTarball/libqxp/configure.ac, and there are many more
workdir/UnpackedTarball/*/m4/ax_gcc_func_attribute.m4 in other external projects,
all of which may cause similar AX_GCC_FUNC_ATTRIBUTE mis-detections.  However,
they do not cause any noticeable traces like the "direct access" ld warning here,
so I left those alone for now.  (Ultimately, all the upstream external projects
should probably deploy the latest version of ax_gcc_func_attribute.m4 from
<http://git.savannah.gnu.org/gitweb/?p=autoconf-archive.git;a=blob_plain;
f=m4/ax_gcc_func_attribute.m4>.)

Change-Id: Ia0560cace770ec7da9ee390566a01a5c592f9209
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/109774
Tested-by: Stephan Bergmann <sbergman@redhat.com>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

external/libcdr/UnpackedTarball_libcdr.mk

@@ -18,6 +18,7 @@ $(eval $(call gb_UnpackedTarball_update_autoconf_configs,libcdr))
 $(eval $(call gb_UnpackedTarball_add_patches,libcdr, \
     external/libcdr/libcdr-visibility-win.patch \
     external/libcdr/libcdr-no-icu-boolean.patch.1 \
+    external/libcdr/ax_gcc_func_attribute.m4.patch \
 ))
 
 # vim: set noet sw=4 ts=4:

external/libcdr/ax_gcc_func_attribute.m4.patch

@@ -0,0 +1,11 @@
+--- configure
++++ configure
+@@ -19167,7 +19167,7 @@
+ 
+ _ACEOF
+ if ac_fn_cxx_try_link "$LINENO"; then :
+-                                      if test -s conftest.err; then :
++                                      if grep -- -Wattributes conftest.err; then :
+   ax_cv_have_func_attribute_visibility=no
+ else
+   ax_cv_have_func_attribute_visibility=yes

external/libqxp/UnpackedTarball_libqxp.mk

@@ -13,8 +13,11 @@ $(eval $(call gb_UnpackedTarball_set_tarball,libqxp,$(QXP_TARBALL)))
 
 $(eval $(call gb_UnpackedTarball_update_autoconf_configs,libqxp))
 
+$(eval $(call gb_UnpackedTarball_set_patchlevel,libqxp,0))
+
 $(eval $(call gb_UnpackedTarball_add_patches,libqxp, \
 	external/libqxp/android-workaround.patch.1 \
+	external/libcdr/ax_gcc_func_attribute.m4.patch \
 ))
 
 # vim: set noet sw=4 ts=4:

external/libqxp/ax_gcc_func_attribute.m4.patch

@@ -0,0 +1,11 @@
+--- configure
++++ configure
+@@ -18136,7 +18136,7 @@
+ 
+ _ACEOF
+ if ac_fn_cxx_try_link "$LINENO"; then :
+-                                      if test -s conftest.err; then :
++                                      if grep -- -Wattributes conftest.err; then :
+   ax_cv_have_func_attribute_visibility=no
+ else
+   ax_cv_have_func_attribute_visibility=yes