make: do assignment and export in a single statement
This is portable between GNU and BSD make, whereas doing the export on
its own line confuses BSD make.
--- a/Makefile Thu Apr 21 10:05:14 2016 -0400
+++ b/Makefile Thu Apr 21 10:10:48 2016 -0400
@@ -4,8 +4,7 @@
#
# % make PREFIX=/opt/ install
-PREFIX=/usr/local
-export PREFIX
+export PREFIX=/usr/local
PYTHON=python
$(eval HGROOT := $(shell pwd))
HGPYTHONS ?= $(HGROOT)/build/pythons