meson: build gallium based osmesa

This has been tested with the osdemo from mesa-demos

v2: - Add SELinux dependency
    - fix typo GALLIUM_LLVM -> GALLIUM_LLVMPIPE

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
diff --git a/meson.build b/meson.build
index ef06ea8..8988dea 100644
--- a/meson.build
+++ b/meson.build
@@ -715,6 +715,9 @@
   if with_osmesa == 'classic' and not with_dri_swrast
     error('OSMesa classic requires dri (classic) swrast.')
   endif
+  if with_osmesa == 'gallium' and not with_gallium_softpipe
+    error('OSMesa gallium requires gallium softpipe or llvmpipe.')
+  endif
   osmesa_lib_name = 'OSMesa'
   osmesa_bits = get_option('osmesa-bits')
   if osmesa_bits != '8'