Compile with -fvisibility-hidden by default

We have all functions that need to be visible marked with PUBLIC and
this is trimming around 4% off the DRI driver .so size.

configure.ac

@@ -95,7 +95,7 @@ esac
 
 dnl Add flags for gcc and g++
 if test "x$GCC" = xyes; then
-    CFLAGS="$CFLAGS -Wall -Wmissing-prototypes -std=c99 -ffast-math"
+    CFLAGS="$CFLAGS -Wall -Wmissing-prototypes -std=c99 -ffast-math -fvisibility=hidden"
 
     # Work around aliasing bugs - developers should comment this out
     CFLAGS="$CFLAGS -fno-strict-aliasing"