Revert "config: remove incorrectly implemented visibility compiler option"

This reverts commit bf9fdceef6.

We really only want to expose symbols that we explicitly mark as part of
the API.  This may not work with other platforms or compilers, but the
fact that private symbols are not available on Linux+GCC is enough of an
incentive to not use those.

Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>

configure.ac

@@ -65,6 +65,10 @@ if test "x$CC_FOR_BUILD" = x; then
     fi
 fi
 
+if  test "x$GCC" = xyes ; then
+  CFLAGS="$CFLAGS -fvisibility=hidden"
+fi
+
 # Obtain protocols headers include directives
 PKG_CHECK_MODULES([X11], [xproto kbproto >= 1.0.5])