* configure.ac: Check gcc and g++ for -fvisibility and
-fvisibility-inlines-hidden.  Add these options to
CFLAGS and CXXFLAGS.
* m4/ax_check_compile_flag.m4: New file.
* src/Makefile.am: Build BuDDy as a single library, reverting part of
the changes introduced in my previous patch to this file.  Since
the options are set in CFLAGS/CXXFLAGS, there is no possibility
for -fvisibility-inlines-hidden to be passed to the C compiler.