* tests/defs (required): Handle `gcc' and `g++'; automatically

* tests/defs (required): Handle `gcc' and `g++'; automatically
export CC and CXX when they are required.
* tests/ansi3.test, tests/ansi5.test, tests/ccnoco.test,
tests/cond4.test, tests/cond16.test, tests/cond18.test,
tests/cond19.test, tests/depcomp2.test, tests/depend2.test,
tests/lex3.test, tests/pr87.test, tests/pr220.test,
tests/substref.test, tests/target-cflags.test, tests/yacc4.test,
tests/yaccvpath.test: Require gcc.  Don't pass CC=gcc to
configure.
* tests/subobj9.test: Require g++.