Allow unbundling OpenXR (for Linux distros)

Copy XrMatrix4x4f_CreateProjectionFov to our OpenXRUtil, instead of relying
on a private header.

(cherry picked from commit 34a07b81ae)

platform/linuxbsd/detect.py

@@ -291,6 +291,9 @@ def configure(env: "Environment"):
         # No pkgconfig file so far, hardcode expected lib name.
         env.Append(LIBS=["embree3"])
 
+    if not env["builtin_openxr"]:
+        env.ParseConfig("pkg-config openxr --cflags --libs")
+
     if env["fontconfig"]:
         if not env["use_sowrap"]:
             if os.system("pkg-config --exists fontconfig") == 0:  # 0 means found