make: silence clang complaining of -pthread

The clang linker doesn't need -pthread and
warns of unknown command-line option.

This detects clang and forgoes the -pthread
flag to the linker.

Change-Id: I658cd887c567a5beeeae0e6e1b6d596231894c1b
Signed-off-by: Ashod Nakashian <ashod.nakashian@collabora.co.uk>

Makefile.am

@@ -83,7 +83,12 @@ if !ENABLE_DEBUG
 AM_CPPFLAGS += -DNDEBUG
 endif
 
-AM_LDFLAGS = -pthread -Wl,-E -lpam $(ZLIB_LIBS) $(ZSTD_LIBS) ${PNG_LIBS}
+AM_LDFLAGS = -Wl,-E -lpam $(ZLIB_LIBS) $(ZSTD_LIBS) ${PNG_LIBS}
+
+# Clang's linker doesn't like -pthread.
+if !HAVE_CLANG
+AM_LDFLAGS += -pthread
+endif
 
 if ENABLE_SSL
 AM_LDFLAGS += ${OPENSSL_LIBS}

configure.ac

@@ -1023,6 +1023,20 @@ else
     AC_MSG_ERROR(Compiler doesn't support C++17)
 fi
 
+# Check if we have clang, rather than gcc, which
+# doesn't like -pthread on the linker command-line.
+AC_MSG_CHECKING([whether the compiler is clang])
+AC_COMPILE_IFELSE([AC_LANG_SOURCE([
+    #if !defined(__clang__)
+    #error not clang
+    #endif
+])],
+                    [AC_MSG_RESULT([yes])
+                    AC_DEFINE([HAVE_CLANG],1,[whether the C++ compiler is clang])],
+                    [AC_MSG_RESULT([no])
+                    AC_DEFINE([HAVE_CLANG],0,[whether the C++ compiler is clang])])
+AM_CONDITIONAL([HAVE_CLANG], [test "$HAVE_CLANG" = "1"])
+
 AS_IF([test "$ENABLE_GTKAPP" != true],
 [CXXFLAGS="$CXXFLAGS $CXXFLAGS_CXXSTD"])
 

test/Makefile.am

@@ -93,7 +93,12 @@ endif
 noinst_LTLIBRARIES = ${all_la_unit_tests}
 
 MAGIC_TO_FORCE_SHLIB_CREATION = -rpath /dummy
-AM_LDFLAGS = -pthread -module $(MAGIC_TO_FORCE_SHLIB_CREATION) $(ZLIB_LIBS) $(ZSTD_LIBS) ${PNG_LIBS}
+AM_LDFLAGS = -module $(MAGIC_TO_FORCE_SHLIB_CREATION) $(ZLIB_LIBS) $(ZSTD_LIBS) ${PNG_LIBS}
+
+# Clang's linker doesn't like -pthread.
+if !HAVE_CLANG
+AM_LDFLAGS += -pthread
+endif
 
 if ENABLE_SSL
 AM_LDFLAGS += ${OPENSSL_LIBS}