We still have checks for $(COM) being GCC or MSC in makefiles, of
course. But there is no reason to pass -D$(COM) to compilations.
Checking the built-in compiler-specific macros __GNUC__, _MSC_VER
(etc) is the right thing to do.

Change-Id: Ia961a29ba74e2c4977e5300a92318f38104c6592