Enable -Wstrict-prototypes and -Wold-style-definition by default

Enable -Wstrict-prototypes and -Wold-style-definition by default

Those are available in all gcc and clang versions that support C11 and as C11
is required as of f5e0186f865c, then we can add them without capability test.

Having them enabled by default avoid having to chase these manually like
11171fe1fc8, cdf4b9aff2, 0e72b9d440, 7069dbcc31, f1283ed6cc, 7b66e2c086,
e95126cf04 and 9f7c527af3 have done.

Also, readline headers trigger a lot of warnings with -Wstrict-prototypes, so
we make use of the system_header pragma to hide the warnings.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/13d51b20-a69c-4ac1-8546-ec4fc278064f%40eisentraut.org
Discussion: https://postgr.es/m/aTFctZwWSpl2/LG5%40ip-10-97-1-34.eu-west-3.compute.internal