Make the Makefile non-mac friendly

Summary:
Add checks in the makefile to not run mac-only things when not on a
mac. (I've only tested on linux, I haven't ensured that this runs mac-only
things on a mac)

Test Plan:
- Run `make`
- Have it not break

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D7260

Makefile

@@ -1,10 +1,17 @@
+UNAME=$(shell uname)
+
 .PHONY: build setup copy serve clean
-build: setup build/katex.js build/katex.less.css pdiff
+build: setup build/katex.js build/katex.less.css
+ifeq ($(UNAME),Darwin)
+build: pdiff
+endif
 
 setup:
 	npm install
+ifeq ($(UNAME),Darwin)
 	brew install webkit2png
 	brew install graphicsmagick
+endif
 
 compress: build/katex.min.js
 	@printf "Minified, gzipped size: "