* ./configure.ac.in: additional warning flags for GCC.
       -Wconversion
           Warn if a prototype causes a type conversion that is different from
           what would happen to the same argument in the absence of a proto-
           type.
       -Wsign-compare
           Warn when a comparison between signed and unsigned values could
           produce an incorrect result when the signed value is converted to
           unsigned.
       -Wunreachable-code
           Warn if the compiler detects that code will never be executed.
       -Wdisabled-optimization
           Warn if a requested optimization pass is disabled.
    See the gcc documentation for more explanations. This triggers a lot of
    warnings everywhere, and at first sight they're all worth fixing. Code
    quality matters, too, so let's fix'em :)