Honor external toolchain environment variables
Honor external toolchain environment variables

If the compiler/CFLAGS were set in the environment, do not override
them.

Signed-off-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

google/u2f-ref-code,google/u2f-ref-code,google/u2f-ref-code,google/u2f-ref-code,google/u2f-ref-code