Allow setting of installation destination via DESTDIR variable in Makefile

This is useful when packaging yosys, as some Linux distributions do not
allow the package management system to install files in /usr/local [1][2].

[1] https://wiki.archlinux.org/index.php/Arch_Packaging_Standards
[2] http://fedoraproject.org/wiki/Packaging:Guidelines

Signed-off-by: Martin Schmölzer <martin.schmoelzer@student.tuwien.ac.at>
Merge pull request #16 from mschmoelzer/master

Allow setting of installation destination via DESTDIR variable in Makefi...