install: stop using current mono prefix to install, respect --prefix arg

Even if the $prefix arg seems completely wrong (as in, there is no mono
installation in that prefix):
- The scripts should respect it (because creating .deb packages involves
installing in sandboxed filesystem paths to extract only the relevant bits
of a tarball.
- Anyway there is already a warning printed by the configure.ac if who is
compiling is a person instead of a buildbot.
- When compiling, proper mono libs of the existing current installation
will be referenced, so building will still work.

This is done by using $gacdir variables instead of the $monogacdir ones
for the install Makefile targets.

This commit finishes DESTDIR support, to work outside of the box, for example
with a .deb package creator like fpm (see a sample script at
https://github.com/knocte/7digital-mono/blob/master/build-fsharp-package.bash ).