Build: configure: some CFLAGS codependent (Wformat-nonliteral+Wformat)

...therefore the extra flags to be tested shall be suitably ordered
+ the support tested in additive manner to prevent false negatives.

Quick demo with GCC7:
$ echo | gcc -E -Werror -Wformat-nonliteral - >/dev/null

cc1: error: -Wformat-nonliteral ignored without -Wformat [-Werror=format-nonliteral]
cc1: all warnings being treated as errors