Porting procmail to C99

Is procmail still widely used?  It has its own non-autoconf build
configuration system, and it's not easily fixed because it assumes that
implicit function declarations are supported: sources with multiple
functions are compiled (but not linked), and those functions that
produce diagnostics are flagged as unsupported (or so I assume).

I'm inclined towards building procmail in C89 mode for Fedora.

Thanks,
Florian