external/hunspell: Work around missing C++20 std::bit_cast

...as witnessed with <https://ci.libreoffice.org/job/gerrit_android_x86/31690/>
on top of patch set 4 of <https://gerrit.libreoffice.org/c/core/+/155121/4>
"Bump baseline to C++20",

> In file included from /home/tdf/lode/jenkins/workspace/android_x86/lingucomponent/source/spellcheck/spell/sspellimp.cxx:35:
> In file included from /home/tdf/lode/jenkins/workspace/android_x86/workdir/UnpackedTarball/hunspell/src/hunspell/hunspell.hxx:74:
> /home/tdf/lode/jenkins/workspace/android_x86/workdir/UnpackedTarball/hunspell/src/hunspell/w_char.hxx:62:17: error: no member named 'bit_cast' in namespace 'std'
>     return std::bit_cast<unsigned short>(*this);
>            ~~~~~^

Change-Id: Ic4a799a0918ff6bcc4dd414d5ae59e02181074bd
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/155346
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

external/hunspell/UnpackedTarball_hunspell.mk

@@ -25,6 +25,7 @@ $(eval $(call gb_UnpackedTarball_add_patches,hunspell, \
 	external/hunspell/0001-fix-LibreOffice-build-problem-with-basic_string-appe.patch \
 	external/hunspell/0001-Resolves-rhbz-2158548-allow-longer-words-for-hunspel.patch \
 	external/hunspell/0001-Keep-only-REP-ph-or-2-word-dictionary-phrase-suggest.patch \
+	external/hunspell/bit_cast.patch.0 \
 ))
 
 # vim: set noet sw=4 ts=4:

external/hunspell/bit_cast.patch.0

@@ -0,0 +1,22 @@
+--- src/hunspell/w_char.hxx
++++ src/hunspell/w_char.hxx
+@@ -42,9 +42,8 @@
+ 
+ #if __cplusplus >= 202002L
+ #include <bit>
+-#else
++#endif
+ #include <cstring>
+-#endif
+ 
+ #ifndef GCC
+ struct w_char {
+@@ -58,7 +57,7 @@
+   {
+ #if defined(__i386__) || defined(_M_IX86) || defined(_M_X64)
+     //use little-endian optimized version
+-#if __cplusplus >= 202002L
++#if __cplusplus >= 202002L && defined __cpp_lib_bit_cast && __cpp_lib_bit_cast >= 201806L
+     return std::bit_cast<unsigned short>(*this);
+ #else
+     unsigned short u;