meson: don't build gallium subdir unless we're building gallium

This will allow us to simplify some guards within the gallium directory.

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>

src/meson.build

@@ -69,7 +69,9 @@ endif
 if with_egl
   subdir('egl')
 endif
-subdir('gallium')
+if with_gallium
+  subdir('gallium')
+endif
 
 # This must be after at least mesa, glx, and gallium, since libgl will be
 # defined in one of those subdirs depending on the glx provider.