Redo the Unix source code generator

Redo the Unix source code generator

For assembler, we want the final target to be foo.s (lowercase s).
However, the build.info may have lines like this (note upper case S):

    GENERATE[foo.S]=foo.pl

This indicates that foo.s (lowercase s) is still to be produced, but
that producing it will take an extra step via $(CC) -E.  Therefore,
the following variants (simplified for display) can be generated:

    GENERATE[foo.S]=foo.pl  =>  foo.s: foo.pl
                                	$(PERL) $foo.pl $@.S; \
                                	$(CC) $(CFLAGS) -E -P $@.S > $@ && \
                                	rm -f $@.S

    GENERATE[foo.s]=foo.pl  =>  foo.s: foo.pl
                                	$(PERL) $foo.pl $@

    GENERATE[foo.S]=foo.m4  =>  foo.s: foo.m4
                                	m4 -B 8192 $foo.m4 > $@.S; \
                                	$(CC) $(CFLAGS) -E -P $@.S > $@ && \
                                	rm -f $@.S

    GENERATE[foo.s]=foo.m4  =>  foo.s: foo.m4
                                	m4 -B 8192 $foo.m4 > $@

Reviewed-by: Andy Polyakov <appro@openssl.org>