CI: Refine C compilers handling. [skip appveyor]

CI: Refine C compilers handling. [skip appveyor]

In build_common.sh add minimal heuristics to print_cc_version() to help
it run the right command; add cc_id() and cc_werr_cflags() to pick the
right CFLAGS for the current compiler instead of the previously
hard-coded "-Werror". Add some comments.

In build.sh remove the CFLAGS exemptions for AIX and Solaris 9 builds,
which are in a better shape now and can complete specific subsets of the
full matrix without a warning. Set CFLAGS from cc_werr_cflags() to make
the best effort to catch as many warnings as possible. Let's see how well
that works.

(cherry picked from commit cdc3bc2359dc214dfedf5fc5dec4712058af7d33)