meson: build softpipe

This doesn't include llvmpipe.

v2: - Fix inconsistent use of with_gallium_swrast and
      with_gallium_softpipe.

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

meson.build

@@ -99,6 +99,7 @@ if _drivers != ''
   _split = _drivers.split(',')
   with_gallium_radeonsi = _split.contains('radeonsi')
   with_gallium_nouveau = _split.contains('nouveau')
+  with_gallium_softpipe = _split.contains('swrast')
   with_gallium = true
   with_dri = true
 endif
@@ -111,6 +112,10 @@ if not (with_dri or with_gallium)
   with_shared_glapi = false
 endif
 
+if with_dri_swrast and with_gallium_softpipe
+  error('Only one swrast provider can be built')
+endif
+
 dep_libdrm_intel = []
 if with_dri_i915
   dep_libdrm_intel = dependency('libdrm_intel', version : '>= 2.4.75')