configure: ensure --enable-omx-rpi uses rpi-specific IL headers

When compiling natively on an RPI where libomxil-bellagio-dev
was also installed, `check_headers OMX_Core.h` succeeded and
the -isystem compiler flag was never added to the build.

For non-native builds, the error message now mentions the
raspberrypi/firmware repository where the RPI specific
headers are available.
Signed-off-by: Aman Gupta <aman@tmm1.net>

configure

@@ -6362,9 +6362,12 @@ enabled opengl            && { check_lib opengl GL/glx.h glXGetProcAddress "-lGL
                                check_lib opengl ES2/gl.h glGetError "-isysroot=${sysroot} -Wl,-framework,OpenGLES" ||
                                die "ERROR: opengl not found."
                              }
-enabled omx_rpi           && { check_headers OMX_Core.h ||
-                               { ! enabled cross_compile && add_cflags -isystem/opt/vc/include/IL && check_headers OMX_Core.h ; } ||
-                               die "ERROR: OpenMAX IL headers not found"; } && enable omx
+enabled omx_rpi           && { test_code cc OMX_Core.h OMX_IndexConfigBrcmVideoRequestIFrame ||
+                               { ! enabled cross_compile &&
+                                 add_cflags -isystem/opt/vc/include/IL &&
+                                 test_code cc OMX_Core.h OMX_IndexConfigBrcmVideoRequestIFrame; } ||
+                               die "ERROR: OpenMAX IL headers from raspberrypi/firmware not found"; } &&
+                             enable omx
 enabled omx               && require_headers OMX_Core.h
 enabled openssl           && { check_pkg_config openssl openssl openssl/ssl.h OPENSSL_init_ssl ||
                                check_pkg_config openssl openssl openssl/ssl.h SSL_library_init ||