Hello-
If I use visibility.m4 on a platform with a recent GCC (4.x) on
Cygwin, the visibility.m4 correctly discovers that gcc can compile
with -fvisibility="hidden". But, on Cygwin (and probably most
non-ELF) gcc does not actualy implement the visibility and emits the
warning "visibility attribute not supported in this configuration"
instead. Even so, the tests in visibility.m4 pass, and
HAVE_VISIBILITY is set in the affirmative.
In my humble opinion, it would be a good idea to add a "-werror" to
the test in visibility.m4 so that it won't set CFLAGS_VISIBILITY and
HAVE_VISIBILITY on those platforms that don't actually support it.