Enable compiling the X Server and modules with hidden symbols by default.

Note that it checks if support for symbol visibility is available by
the compiler, not if it is functional. It may have problems on non x86
architectures.
  To disable the feature, just pass the option "--disable-visibility"
to the X Server configure script.
  Unless using an alternate build schema, drivers built from a git
checkout will use the same "visibility" related CFLAGS used to compile
the X Server.

xorg-server.pc.in

@@ -15,5 +15,5 @@ Name: xorg-server
 Description: Modular X.Org X Server
 Version: @PACKAGE_VERSION@
 Requires: pixman-1 pciaccess
-Cflags: -I${sdkdir}
+Cflags: -I${sdkdir} @symbol_visibility@
 Libs: -L${libdir}