configure: use ac_c_werror_flag to force unknown compiler flags to fail.

It's not a documented feature, but it's what the documented
AC_LANG_WERROR has used for 13 years, and there's no push/pop mechanism
for AC_LANG_WERROR, so you can't ensure that "fail even on warnings"
will be applied *only* in AC_LBL_CHECK_COMPILER_OPT(), as that's what we
want.  (If we can make sure that *no* compiler tests will produce
warnings, except for the ones we *want* to fail if they produce
warnings, we could just do AC_LANG_WERROR, but that might be tricky to
ensure in the general case.)

We do this because not all compilers have a command-line flag to force
all warnings, *including* warnings from unknown commad-line flags (I'm
looking at *you* IBM XL C!), so we have to have the test check to make
sure no warnings are produced (which, for AC_TRY_COMPILE(), means
"nothing is written to the standard output").

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