Instruct configure to hide internal libgc.so symbols if supported (GCC)

Instruct configure to hide internal libgc.so symbols if supported (GCC)

* configure.ac (ac_cv_fvisibility_hidden): New variable (set to "yes"
if only shared library is built and GCC supports "-fvisibility" option.
* configure.ac (CFLAGS): Add "-fvisibility=hidden" and
"-DGC_VISIBILITY_HIDDEN_SET" if ac_cv_fvisibility_hidden is "yes".
* include/gc_config_macros.h (GC_API): Force "default visibility"
attribute if GC_VISIBILITY_HIDDEN_SET (only if GCC and GC_BUILD).
* tests/staticrootstest.c (GC_VISIBILITY_HIDDEN_SET): Test the macro
and skip the test if defined (since "staticrootslib" symbols are of
hidden visibility).