build: expand ~ in install prefix early

The install prefix gets written to config.gypi and config.mk.  Tildes
were expanded in the first file but not in the second one, causing the
`make install` target to install files to a directory named `~` in
the current working directory.

Fixes: https://github.com/nodejs/node/issues/75
PR-URL: https://github.com/nodejs/io.js/pull/2307


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>