Enable -fvisibility=hidden on Linux.

This will prevent further symbol clashes, and makes the shared libraries
 10 to 20 percent smaller.

We should enable this on other platforms, too, if we can guarantee they'll
 use gcc 4.0 or later and the platform supports it.

code/game/g_syscalls.c

@@ -31,7 +31,7 @@ Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 static intptr_t (QDECL *syscall)( intptr_t arg, ... ) = (intptr_t (QDECL *)( intptr_t, ...))-1;
 
 
-void dllEntry( intptr_t (QDECL *syscallptr)( intptr_t arg,... ) ) {
+Q_EXPORT void dllEntry( intptr_t (QDECL *syscallptr)( intptr_t arg,... ) ) {
 	syscall = syscallptr;
 }