configure: add macro to check compiler flag '-Wstrict-prototype'

configure: add macro to check compiler flag '-Wstrict-prototype'

Strict prototype are better for portability and to avoid bugs because it
makes sure the compiler has the information to properly validate the
arguments given when a function is called.

This flag however need special care when checking for it, because the
declaration for 'main' generated by autoconf cannot be a strict prototype
so the detection would always see the flag as failing.

This patch handles this by creating a dedicated macro for this detection
which uses a good prototype because we're in a case where it is possible,
so the detection will not always fail; it also makes sure to add the flag
to CFLAG only at the end, to avoid falsely crashing any further test done
in the configure script.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>