visibility.m4

[Mike Gran]

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.

What do you think?

Thanks

-Mike Gran