BUILD: respect DESTDIR variable

DESTDIR is used for temporary install location
in package managers otherwise you have to add
the temporary locations to BINDIR and LIBDIR
which can cause trouble with the pkg-config file.

The adding of .PHONY caused a silent regression for us, we can just sed it out, but respecting DESTDIR variable will allow a better solution.
see https://bugs.gentoo.org/show_bug.cgi?id=477948

Also: make does not take care of whitespaces in $(VAR)