configure.ac: change our visibility policy

configure.ac: change our visibility policy

Check for -fvisibility=hidden as a supported CFLAG.

If it is supported, use it and emit an AC_DEFINE to change the meaning
of _GLIB_EXTERN to include the GNU attribute for marking symbols as
public: __attribute((visibility("default"))).

This will override the public definition of _GLIB_EXTERN for any file
which does #include "config.h" (forcing all our .c files to do so, as a
side effect).

If we're on mingw, assume that -fvisibility will work and also throw in
a __declspec(dllexport) for good measure.  This will allow us to move
away from using a .def file to create the the various DLLs.

It's possible that there may be compilers that accept
-fvisibility=hidden but don't accept the GNU attribute for making
symbols public again -- we will hopefully receive bugs if any of those
exist.

https://bugzilla.gnome.org/show_bug.cgi?id=688681