Update nasm.spec.in to match modern conventions, and make it handle rc
releases by using the "mangled" version of the name (1.99.99.91
instead of 2.0rc1).
Support version numbers of the form X.Y[.Z]rcW where X, Y, Z and W are
numbers. For the numeric macros, drop them down to a lower level, so
2.0rc1 is treated as version 1.99.99.91.