Add a helper Makefile for common tasks

It is ignored for git exports, because it uses git internally and is
mostly useful for developers, who will work with a git checkout anyway.

.gitattributes

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+
+# ignore Makefile for distribution archives
+/Makefile export-ignore

.gitignore

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+
+# distribution archives
+easyrpg-rtp.zip
+easyrpg-rtp.tar.gz

Makefile

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+
+# helper variable
+MYDIR:=$(shell dirname $(realpath $(lastword $(MAKEFILE_LIST))))
+
+help:
+	@echo "This Makefile is intended to be used by developers only."
+	@echo "Therefore it does nothing by default. Available targets:"
+	@echo "    optimize     :"
+	@echo "            -png : Recompresses png files with zopfli for smaller file size"
+	@echo "    dist         : "
+	@echo "        -zip     : Creates a distribution zip archive (for use under Windows)"
+	@echo "        -tar     : Creates a distribution tarball (for other platforms)"
+	@echo "The meta targets will execute all their subtargets."
+
+dist: dist-tar dist-zip
+
+dist-zip:
+	@rm -f easyrpg-rtp.zip
+	@git archive --prefix=EasyRPG-RTP/ -9 --output=$(MYDIR)/easyrpg-rtp.zip master
+
+dist-tar:
+	@rm -f easyrpg-rtp.tar.gz
+	@git archive --prefix=EasyRPG-RTP/ --output=$(MYDIR)/easyrpg-rtp.tar.gz master
+
+optimize: optimize-png
+
+optimize-png:
+	find $(MYDIR) -iname "*.png" -execdir advpng -z4 {} \+