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.

tests/local.at

@@ -634,4 +634,15 @@ m4_popdef([AT_COND_CASE])])
 
 AT_INIT
 
+# Cannot assign CC and CFLAGS here, since atlocal is loaded after
+# options are processed, so we don't know the value of CXX and
+# CXXFLAGS yet.
+#
+# Note that it also means that command line values for CXX and
+# CXXFLAGS will not be propagated to CC and CFLAGS.
+AT_ARG_OPTION([compile-c-with-cxx],
+              [compile C parsers with the C++ compiler])
+
+AT_COLOR_TESTS
+
 AT_TESTED([bison])