build: rename some build-time variables, for consistency

Before this change, the variables used for a build configured with the
prefix "/usr/local", and where Mono was installed in "/usr" were:

* $prefix               =>      /usr/local
* $libdir               =>      /usr/local/lib
* $monoprefix           =>      /usr
* $monorootdir          =>      /usr/lib
* $monodir              =>      /usr/lib/mono
* $monolibdir2          =>      /usr/lib/mono/2.0
* $monolibdir35         =>      /usr/lib/mono/3.5
* $monolibdir4          =>      /usr/lib/mono/4.0
* $monolibdir           =>      $monolibdir{2|35|4}

From now on, we'll have this naming which is a bit more consistent, and
allows to target the /mono/ subdirectory in the targeted prefix (as opposed
to the /mono/ directory in the prefix of the currently installed Mono):

* $prefix               =>      /usr/local
* $libdir               =>      /usr/local/lib
* $monoprefix           =>      /usr
* $monolibdir           =>      /usr/lib
* $monogacdir           =>      /usr/lib/mono
* $monogacdir20         =>      /usr/lib/mono/2.0
* $monogacdir35         =>      /usr/lib/mono/3.5
* $monogacdir40         =>      /usr/lib/mono/4.0
* $monogacdirXX         =>      $monogacdir{20|35|40}
* $gacdir               =>      /usr/local/lib/mono
* $gacdir20             =>      /usr/local/lib/mono/2.0
* $gacdir35             =>      /usr/local/lib/mono/3.5
* $gacdir40             =>      /usr/local/lib/mono/4.0
* $gacdirXX             =>      $gacdir{20|35|40}

This cleanup doesn't change behaviour, but improves the field, in
preparation for fixage of issue #154 "DESTDIR to be respected"
(next commit).