Make _AC_C_C99_TEST_MAIN more robust with -Werror

Make _AC_C_C99_TEST_MAIN more robust with -Werror

The current code fails with gcc r16-2729-g0d276cd378e and
  -Werror=discarded-qualifiers -Werror=write-strings
(the -Werror=write-strings is useful to check compatibility of
user code with C++, and the -Werror=discarded-qualifiers is an
upgrade of a default warning to an error). Since this is a test
for C99, we may assume that const is supported by the compiler,
and adding const makes the code more robust in this context.

Copyright-paperwork-exempt: yes