Enable -fvisibility-inlines-hidden

All inlined class member functions are then hidden by default,
leading to potentially smaller binary sizes and shorter link times.

Without the flag, libaudcore.so for example included a symbol for
QEvent::setAccepted() from /usr/include/qt/QtCore/qcoreevent.h:

$ nm -D libaudcore.so
0000000000035f50 W _ZN6QEvent11setAcceptedEb

See also: https://gcc.gnu.org/wiki/Visibility

Author: radioactiveman

acinclude.m4

@@ -151,7 +151,7 @@ if test "x$HAVE_MSWINDOWS" = "xyes" ; then
     EXPORT="__declspec(dllexport)"
 elif test "x$GCC" = "xyes" ; then
     CFLAGS="$CFLAGS -fvisibility=hidden"
-    CXXFLAGS="$CXXFLAGS -fvisibility=hidden"
+    CXXFLAGS="$CXXFLAGS -fvisibility=hidden -fvisibility-inlines-hidden"
     EXPORT="__attribute__((visibility(\"default\")))"
 else
     AC_MSG_ERROR([Unknown syntax for EXPORT keyword])

meson.build

@@ -73,7 +73,8 @@ if cc.get_id() in ['gcc', 'clang']
 
   cxx_flags = [
     '-Wno-non-virtual-dtor',
-    '-Woverloaded-virtual'
+    '-Woverloaded-virtual',
+    '-fvisibility-inlines-hidden'
   ]
 
   check_cflags = common_flags