meson: build gbm

This doesn't include egl support, just dri support.

v2: - when gbm is set to 'auto', only build if a dri driver is also
      enabled
    - Fix conditional to check for x11 modules with vulkan as well as
      with dri drivers
v3: - Set pkgconfig libraries.private value

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

meson.build

@@ -101,12 +101,15 @@ pre_args += '-DHAVE_LIBDRM'
 with_dri2 = with_dri_platform == 'drm' and dep_libdrm.found()
 with_dri3 = get_option('dri3')
 if with_dri3 == 'auto'
-  if host_machine.system() == 'linux'
+  if host_machine.system() == 'linux' and with_dri2
     with_dri3 = true
   else
     with_dri3 = false
  endif
 elif with_dri3 == 'yes'
+  if not with_dri2
+    error('dri3 support requires libdrm')
+  endif
   with_dri3 = true
 else
   with_dri3 = false
@@ -122,6 +125,18 @@ if _platforms != ''
   with_platform_wayland = _split.contains('wayland')
 endif
 
+with_gbm = get_option('gbm')
+if with_gbm == 'auto' and with_dri  # TODO: or gallium
+  with_gbm = host_machine.system() == 'linux'
+elif with_gbm == 'yes'
+  if not ['linux', 'bsd'].contains(host_machine.system())
+    error('GBM only supports unix-like platforms')
+  endif
+  with_gbm = true
+else
+  with_gbm = false
+endif
+
 with_glx = get_option('glx')
 if with_glx != 'disabled'
   pre_args += '-DGLX_USE_TLS'