Fix EGL detection on RPi

config.tests/EGL/EGL.pro

@@ -1,4 +1,13 @@
 SOURCES = main.cpp
 
 CONFIG += link_pkgconfig
+
+# This adds /opt/vc/include to the include path which
+# pulls in the BRCM GLES and EGL libraries. If we don't
+# add this, we'll get the system's headers which expose
+# functionality the runtime GL implementation won't have.
+packagesExist(mmal) {
+    PKGCONFIG += mmal
+}
+
 PKGCONFIG += sdl2 egl