plug-ins: in pygimp/plug-ins/Makefile.am, re-copy plug-ins upon change

In pygimp/plug-ins/Makefile.am, fix the rule for the generated
plug-in files, so that they're re-copied to the build dir upon
changes to the source plug-in files.

(cherry picked from commit 95044d86b3)

plug-ins/pygimp/plug-ins/Makefile.am

@@ -41,8 +41,12 @@ test_scripts = \
 	sphere/sphere.py						\
 	whirlpinch/whirlpinch.py
 
-%.py:
-	mkdir -p $(@D) && cp -f $(srcdir)/$(@F) $@
+$(scripts) $(test_scripts): $(source_scripts)
+	@							\
+	dir=`dirname "$@"`;					\
+	base=`basename "$@"`;					\
+	echo "  GEN      $@";					\
+	mkdir -p "$$dir" && cp -f "$(srcdir)/$$base" "$@";
 
 nobase_pluginexec_SCRIPTS = $(scripts)