fix clens include path for nonstandard prefix, take 2

Change substituted prefix to point to brew --prefix. Also substitute
path in GNUmakefile; that's the file that's actually used in my build
environment. Keep the original Makefile change since it seems like it
will work with non-GNU make.

Please see https://github.com/Homebrew/homebrew/issues/28641
for details on the build bug.