meson: Correct behavior of vdpau=auto

Currently if vdpau is set to auto, it will be disabled only in cases
where gallium is disabled or the host OS is not supported (mac, haiku,
windows). However on (for example) Linux if libvdpau is not installed
then the build will error because of the unmet dependency. This corrects
auto to do the right thing, and not error if libvdpau is not installed.

Fixes: 992af0a4b8224bdb4809e01c2f00d2f32546aee5
       ("meson: dedup gallium-vdpau logic")
Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
diff --git a/meson.build b/meson.build
index a2d5977..429865c 100644
--- a/meson.build
+++ b/meson.build
@@ -398,16 +398,17 @@
   else
     _vdpau = 'false'
   endif
-elif _vdpau == 'auto'
-  _vdpau = 'true'
 endif
-with_gallium_vdpau = _vdpau == 'true'
 dep_vdpau = null_dep
-if with_gallium_vdpau
-  dep_vdpau = dependency('vdpau', version : '>= 1.1')
-  dep_vdpau = declare_dependency(
-    compile_args : run_command(prog_pkgconfig, ['vdpau', '--cflags']).stdout().split()
-  )
+with_gallium_vdpau = false
+if _vdpau != 'false'
+  dep_vdpau = dependency('vdpau', version : '>= 1.1', required : _vdpau == 'true')
+  if dep_vdpau.found()
+    dep_vdpau = declare_dependency(
+      compile_args : run_command(prog_pkgconfig, ['vdpau', '--cflags']).stdout().split()
+    )
+    with_gallium_vdpau = true
+  endif
 endif
 
 if with_gallium_vdpau