Do not allow VERSION to be overloaded by build system.

When integrating libuEv as a component in another piece of software,
that build system may set VERSION as an environment variable.  We do
not want that version to be set as the version of libuEv.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

Makefile

@@ -27,7 +27,7 @@
 ROOTDIR    ?= $(shell pwd)
 
 #VERSION    = $(shell git tag -l | tail -1)
-VERSION    ?= 1.2.3
+VERSION     = 1.2.3
 NAME        = libuev
 PKG         = $(NAME)-$(VERSION)
 ARCHIVE     = $(PKG).tar.xz