Make generation of dependency files more efficient when possible

When building with GNU C, clang or VMS C, it's more efficient to
generate dependency file and object file in one call rather than two.
Have the dependency output in a temporary file and compare it with the
previous one if available to see if replacement is waranted, thereby
avoiding unnecessary reconstruction of Makefile / descrip.mms.

Github issue #750
Reviewed-by: NRich Salz <rsalz@openssl.org>

Configurations/descrip.mms.tmpl

@@ -435,19 +435,15 @@ configdata.pm : {- sourcefile("Configurations", "descrip.mms.tmpl") -} $(SRCDIR)
       my $after = $unified_info{after}->{$obj.".OBJ"} || "\@ !";
 
       return <<"EOF";
-$obj.MMS : $deps
+$obj.OBJ : $deps
         ${before}
         SET DEFAULT $forward
-        \$(CC) \$(CFLAGS)${incs} /MMS=(FILE=${objd}${objn}.MMS,TARGET=$obj.OBJ) /NOOBJECT $srcs
-        SET DEFAULT $backward
-        ${after}
-        - PURGE $obj.MMS
-$obj.OBJ : $obj.MMS
-        ${before}
-        SET DEFAULT $forward
-        \$(CC) \$(CFLAGS)${incs} /OBJECT=${objd}${objn}.OBJ /REPOSITORY=$backward $srcs
+        \$(CC) \$(CFLAGS)${incs} /MMS=(FILE=${objd}${objn}.tmp-MMS,TARGET=$obj.OBJ) /OBJECT=${objd}${objn}.OBJ /REPOSITORY=$backward $srcs
         SET DEFAULT $backward
         ${after}
+        \@ PIPE ( \$(PERL) -e "use File::Compare qw/compare_text/; my \$x = compare_text(""$obj.MMS"",""$obj.tmp-MMS""); exit(0x10000000 + (\$x == 0));" || -
+                 RENAME $obj.tmp-MMS $obj.mms )
+        \@ IF F\$SEARCH("$obj.tmp-MMS") .NES. "" THEN DELETE $obj.tmp-MMS;*
         - PURGE $obj.OBJ
 EOF
   }

Configurations/unix-Makefile.tmpl

@@ -847,11 +847,12 @@ $obj$objext: $obj$depext
 EOF
       }
       return <<"EOF";
-$obj$depext: $deps
-	\$(CC) \$(CFLAGS) $ecflags$incs -MM -MF \$\@ -MQ $obj$objext $srcs
-	touch \$\@
-$obj$objext: $obj$depext
-	\$(CC) \$(CFLAGS) $ecflags$incs -c -o \$\@ $srcs
+$obj$objext: $deps
+	\$(CC) \$(CFLAGS) $ecflags$incs -MMD -MF $obj$depext.tmp -MT \$\@ -c -o \$\@ $srcs
+	\@touch $obj$depext.tmp
+	\@if ! cmp $obj$depext.tmp $obj$depext > /dev/null 2> /dev/null; then \\
+		mv $obj$depext.tmp $obj$depext; \\
+	fi
 EOF
   }
   # On Unix, we build shlibs from static libs, so we're ignoring the