Change the mtc Makefile to check $SDKROOT value instead

of depending on it being set in the environment.  Fred's change
from October assumed that SDKROOT was set in the environment
so that 'xcrun --show-sdk-path' would print the path.  If it
was passed in as a Makefile variable, it wouldn't be set in
the environment and xcrun --show-sdk-path would always show the
macOS SDK path.  When running the lldb testsuite against an ios
device via lit, this seems to be the case.