fix build problems on linux

Makefile

@@ -8,7 +8,7 @@ all: build minify
 
 build: $(LIB)
 	@npm run build
-	@sed -i "" $$'s/\t/  /' $(TARGET)
+	@./tools/tab2spaces
 
 minify: $(TARGET)
 	@npm run minify

tools/tab2spaces

@@ -0,0 +1,16 @@
+#!/usr/bin/env node
+
+const fs = require('fs');
+const path = require('path');
+const target = path.join(__dirname, '../css-doodle.js');
+
+try {
+  const content = fs.readFileSync(target, 'utf8')
+    .replace(/\t/g, '  ')
+
+  if (content) {
+    fs.writeFileSync(target, content, 'utf8');
+  }
+} catch (e) {
+  // it's ok
+}