make: backout changeset 51f5fae84e43

Support for '!=' was only added in GNU Make 4.0, and CentOS versions as new as
CentOS 7 only carry 3.82.

I will leave figuring out compatibility with BSD make as an exercise for
interested folks.

Makefile

@@ -6,7 +6,7 b''
 
 export PREFIX=/usr/local
 PYTHON=python
-HGROOT != pwd
+$(eval HGROOT := $(shell pwd))
 HGPYTHONS ?= $(HGROOT)/build/pythons
 PURE=
 PYFILES:=$(shell find mercurial hgext doc -name '*.py')