perl: detect new files in MakeMaker builds

While Makefile.PL now finds .pm files on its own, it does not
detect new files after it generates perl/perl.mak.

[ew: commit message, minor tweaks]

ref: http://mid.gmane.org/7vlii51xz4.fsf@alter.siamese.dyndns.org

Signed-off-by: Eric Wong <normalperson@yhbt.net>

Makefile

@@ -2090,6 +2090,13 @@ $(SCRIPT_LIB) : % : %.sh GIT-SCRIPT-DEFINES
 ifndef NO_PERL
 $(patsubst %.perl,%,$(SCRIPT_PERL)): perl/perl.mak
 
+perl/perl.mak: perl/PM.stamp
+
+perl/PM.stamp: FORCE
+	$(QUIET_GEN)find perl -type f -name '*.pm' | sort >$@+ && \
+	{ cmp $@+ $@ >/dev/null 2>/dev/null || mv $@+ $@; } && \
+	$(RM) $@+
+
 perl/perl.mak: GIT-CFLAGS GIT-PREFIX perl/Makefile perl/Makefile.PL
 	$(QUIET_SUBDIR0)perl $(QUIET_SUBDIR1) PERL_PATH='$(PERL_PATH_SQ)' prefix='$(prefix_SQ)' $(@F)
 

perl/.gitignore

@@ -5,3 +5,4 @@ MYMETA.yml
 blib
 blibdirs
 pm_to_blib
+PM.stamp

perl/Makefile

@@ -20,6 +20,9 @@ clean:
 	$(RM) ppport.h
 	$(RM) $(makfile)
 	$(RM) $(makfile).old
+	$(RM) PM.stamp
+
+$(makfile): PM.stamp
 
 ifdef NO_PERL_MAKEMAKER
 instdir_SQ = $(subst ','\'',$(prefix)/lib)