Teach Configure and cflags.SH about C99

Teach Configure and cflags.SH about C99
Test declarations after statement in Configure's C99 probe code

Probe to see whether we need -std=gnu99 or -std=c99 to get C99 code to
compile. In cflags.SH, remove code that added gcc warning flags that were
compatible with C89 but are not compatible with C99.

Also ensure that the relevant failure error message is output even with
Configure's -s flag, as we shouldn't stay silent for a message that causes
Configure default to aborting.

With these changes, Configure will fail the C99 probe test if passed
-Accflags="-Werror=declaration-after-statement" or
-Accflags="-Werror=long-long"

Backport of 0c7be120244c1a6aae9b1ae30124265bcb8184da
        and d34aca5e9e57e3f0b5d25d451759237b40647664