vc4: Only build in simulator mode if we find pkg-config for it.

This will let other developers build it x86 for build-testing purposes.
diff --git a/configure.ac b/configure.ac
index 71c9b6c..1ef5fbc 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2161,11 +2161,8 @@
             gallium_require_drm "vc4"
             gallium_require_drm_loader
 
-            case "$host_cpu" in
-                i?86 | x86_64 | amd64)
-                USE_VC4_SIMULATOR=yes
-                ;;
-            esac
+            PKG_CHECK_MODULES([SIMPENROSE], [simpenrose],
+                              [USE_VC4_SIMULATOR=yes], [USE_VC4_SIMULATOR=no])
             ;;
         *)
             AC_MSG_ERROR([Unknown Gallium driver: $driver])