don't allow overriding the path to zsh

Makefile

@@ -1,7 +1,7 @@
-ZSH ?= $(shell command -v zsh 2> /dev/null)
+ZSH := $(shell command -v zsh 2> /dev/null)
 
 all:
 
 pkg:
-	cd gitstatus && $(MAKE) ZSH=$(ZSH) pkg && cd ..
+	cd gitstatus && $(MAKE) pkg && cd ..
 	$(or $(ZSH),:) -fc 'for f in *.zsh-theme internal/*.zsh; do zcompile -R -- $$f.zwc $$f || exit; done'