Add Linux compatibility

Author: kruton

Diff for: Makefile

@@ -1,8 +1,19 @@
-GIMP=/Applications/Gimp.app/Contents/MacOS/gimp-2.8
+PLATFORM ?= $(shell uname)
 
-ASSETS=$(wildcard assets/*.png)
-SRCS=$(wildcard text/*.txt)
-OUTPUT=$(SRCS:text/%.txt=output/%.png)
+ifeq ($(PLATFORM),Linux)
+    GIMP ?= $(shell which gimp)
+endif
+ifeq ($(PLATFORM),Darwin)
+    GIMP ?= $(wildcard /Applications/Gimp.app/Contents/MacOS/gimp-*)
+endif
+
+ifeq ($(wildcard $(GIMP)),)
+    $(error Gimp must be installed. Set GIMP variable to executable path.)
+endif
+
+ASSETS := $(wildcard assets/*.png)
+SRCS := $(wildcard text/*.txt)
+OUTPUT := $(SRCS:text/%.txt=output/%.png)
 
 output/%.png: text/%.txt src/create-play-promo.scm $(ASSETS)
 	echo '(localize-subheading "$(shell cat $<)" "Essential PragmataPro" "$@") (gimp-quit 0)' | cat src/create-play-promo.scm - | $(GIMP) -n -i -b -