Allow to specify a custom install directory

[emersion]

This paves the way to packaging. The variable DESTDIR is the "standard" installation directory variable for Makefiles.

This PR doesn't attempt to change some questionable behaviour of the installation such as:

• Copying the whole lem source directory to /usr/share/lem (why???)
• Using recipes for installed files
• Using cp instead of install

[fshaked]

We have a working opam package and we will make it public soon (see the opam-packaging branch).