test defs: setup `*FLAGS' variables for GNU compilers

test defs: setup `*FLAGS' variables for GNU compilers

* configure.ac: Setup some `*FLAGS' variables for use by the
GNU compilers in our testsuite.  For example, use `GNU_CFLAGS'
instead of `CFLAGS', and so on for similar variables.  This
is especially useful in case the compilers found or defined
at configure time are not the GNU ones.
* tests/defs-static.in: Initialize those same variables with
the values set at configure time, but allowing overrides
from the environment.
* tests/Makefile.am (do_subst): Process configure-style
substitutions of those variables (e.g., `@GNU_CFLAGS@').
* tests/defs: When a GNU compiler is required, override the
corresponding generic `*FLAGS' variable with the GNU-specific
variant (e.g., redefine `$CFLAGS' to take the value of
`$GNU_CFLAGS').