The following variables control the compiler command line:
CPU_SOURCE
The source CPU type is added as a define to the compiler command line. This is determined by the Makefile itself.
CPU_TARGET
The target CPU type is added as a define to the compiler command line. This is determined by the Makefile itself.
OS_SOURCE
What platform the makefile is used on. Detected automatically.
OS_TARGET
What platform will be compiled for. Added to the compiler command line with a -T prepended.