build: allow custom PackageMaker path

Make PACKAGEMAKER customizable because PackageMaker is not necessarily
installed in /Developer on OSX anymore.

PR: nodejs#9377
PR-URL: nodejs#9377
Reviewed-By: Timothy J Fontaine <tjfontaine@gmail.com>

Makefile

@@ -223,7 +223,7 @@ TARBALL=$(TARNAME).tar.gz
 BINARYNAME=$(TARNAME)-$(PLATFORM)-$(ARCH)
 BINARYTAR=$(BINARYNAME).tar.gz
 PKG=out/$(TARNAME).pkg
-packagemaker=/Developer/Applications/Utilities/PackageMaker.app/Contents/MacOS/PackageMaker
+PACKAGEMAKER ?= /Developer/Applications/Utilities/PackageMaker.app/Contents/MacOS/PackageMaker
 
 PKGSRC=nodejs-$(DESTCPU)-$(RAWVER).tgz
 ifdef NIGHTLY
@@ -273,7 +273,7 @@ $(PKG): release-only
 		-create
 	mv $(PKGDIR)/usr/local/bin/node-universal $(PKGDIR)/usr/local/bin/node
 	rm -rf $(PKGDIR)/32
-	$(packagemaker) \
+	$(PACKAGEMAKER) \
 		--id "org.nodejs.Node" \
 		--doc tools/osx-pkg.pmdoc \
 		--out $(PKG)