tests: when using the C++ compiler, use its flags too.

tests: when using the C++ compiler, use its flags too.

* tests/local.at: Go for colors.
(--compile-c-with-cxx): New option.
We used to pass "CC=$CXX" as command line argument,
but it was not possible to adjust CFLAGS accordingly
in atlocal, since it is loaded before assignments on
the command line are honored (so that the command line
takes precedence).
* tests/atlocal.in: Implement it.
* tests/local.mk: Use it.