Add warnings directly to CFLAGS.

The automake docs say passing -Wall etc. to automake options
will pass them on to the compiler command line, but it doesn't
seem to work. Instead add them directly.

We test that the compiler doesn't reject the new flags to
avoid breaking the build with older tools.
Ask for -Wall -Wextra.
Apply extra warning flags unconditionally.

I can't find a compiler to test the conditional warning
flags on. Even tcc just ignore -W flags, and the rest
of our build doesn't work with it. Therefore it's not
worth carrying the code to check.
Merge pull request #284 from rillian/warning

Enabled warnings on the autoconf build.