Improve str.gob testcase on non-GNU compilers.

Checking for -Wformat warnings from the compiler is not going to work
well on non-GNU toolchains, or even on old GNU toolchains where the
warning is not enabled by default, or if the user specified something
like -w in CFLAGS.

Adapt the test to check whether the compiler produces such warnings
and produce a skipped result if not.