Check more Make variables

Some more common makevars methods were required in order to remove the
"-isystem" prefixes from the toolchain.IncludeFlags() value. In Make,
the -isystem is prepended at time of use, not in TARGET_C_INCLUDES
itself.

Change-Id: If07e69ddb7357d11c7dd48ab60f503d219f29de8
2 files changed