Checking all C warning flags at once during configuration

[Alex Gramiak]

There was recently a discussion about the long configuration step of the
build process, and I noticed that a non-negligible section involved
checking support for specific C warning flags.

Assuming that most/all of the warnings are supported by common modern
compilers, would it make sense to first check _all_ (or, at least the
commonly supported) warning flags simultaneously and then only check
them individually if there was an error?

This way the common behaviour would be sped up a bit, and the case of
unsupported flags would just be an extra AC_LINK_IFELSE slower.