Pass -D GC_DLL -fvisibility=hidden if default configure build is requested

Pass -D GC_DLL -fvisibility=hidden if default configure build is requested
(fix of commit c9964cfc0)

If --disable-shared and --enable-static are not passed to configure
then build generates only shared libraries, thus the internal symbols
visibility could be restricted.

* configure.ac [$enable_shared="" && $enable_static!=yes
|| $enable_shared=yes && $enable_static==""] (GC_DLL): Define AC macro.
* configure.ac [($enable_shared="" && $enable_static!=yes
|| $enable_shared=yes && $enable_static=="") && $GCC=yes
&& $ac_cv_fvisibility_hidden=yes] (CFLAGS): Add
-DGC_VISIBILITY_HIDDEN_SET -fvisibility=hidden.