amd-gpu-x11-bin-mx51: Proper detect the building on Linux

amd-gpu-x11-bin-mx51: Proper detect the building on Linux

GCC defines __linux__ when running on Linux but the headers where
checking by _LINUX define hence enforcing explicit definition by build
system; we patch the headers to check for __linux__ and then ensure it
"Just Work (TM)".

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Acked-by: Rogerio Pimentel <rogerio.pimentel@freescale.com>