warnings.m4: check the compiler, not the preprocessor.

[Akim Demaille]

For Bison I would like to test C and C++ warnings
in two different batches.  Currently the macro does
not depend on the language to store its result, and
it does not try to compile, just to preprocess.

0001-warnings.m4-check-the-compiler-not-the-preprocessor.patch
Description: Binary data

FWIW I used to use a generic scheme, where I checked the
stderr of the compiler to see if the option was really
accepted: in some case (e.g., GCC) the compiler will
accept the option ($? = 0) yet it will constantly complain
about its uses.  Is there interest in this approach?
Attached for the records.

warning.m4
Description: Binary data