tests: beware of compilers that do not support POSIXLY_CORRECT

tests: beware of compilers that do not support POSIXLY_CORRECT

Running "maintainer-release-check" on OS X with Clang 2.9 fails,
because "clang-mp-2.9 -o test -g test.c" launches "/usr/bin/dsymutil
test -o test.dSYM" which fails with "error: unable to open executable
'-o'".

* m4/c-working.m4 (BISON_CHECK_WITH_POSIXLY_CORRECT)
(BISON_C_COMPILER_POSIXLY_CORRECT): New.
* configure.ac: Use the latter.
* tests/atlocal.in (POSIXLY_CORRECT_IS_EXPORTED): New.
* tests/local.at (AT_BISON_CHECK_WARNINGS_): Use it instead of computing its
value each time.
(AT_QUELL_VALGRIND): Skip tests that cannot work because of compilers
that do not support POSIXLY_CORRECT.