Add a Makefile.nmake.inc file, to hold rules etc. used by multiple

Add a Makefile.nmake.inc file, to hold rules etc.  used by multiple
Makefile.nmake files; currently, it has the (F)lex-to-C rule and a
.SUFFIXES pseudo-rule to add .l to the list of suffixes.  Have
Makefile.nmake files with .l.c rules include Makefile.nmake.inc to get
that rule.

The names Makefile.am.inc and Makefile.nmake.inc aren't necessarily the
right names for the files in question.

Use $(PACKAGE) in the Mate plugin's Makefile, rather than "mate".

svn path=/trunk/; revision=22437