Add support for daily snapshot releases of the form
<ordinary version number>-<datecode>. These are exported to programs
as the new macro __NASM_SNAPSHOT__ (only present in snapshot releases.)
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.
