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>