configure: check for -fvisibility=hidden
configure: check for -fvisibility=hidden

and make it the default if available. this limits symbol visibility to
those marked with WEBP_EXTERN.

Change-Id: I529bf6d143a3008d9385044c87ad7dce72dce9a0

PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,ttyangf/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,ttyangf/libwebp,PKRoma/libwebp,webmproject/libwebp,ttyangf/libwebp,PKRoma/libwebp,ttyangf/libwebp,ttyangf/libwebp,PKRoma/libwebp,webmproject/libwebp