support DESTDIR and prefix installation

[realtime-neil]

The Makefile does not support a configurable installation destination. It should be fixed to support both DESTDIR and prefix as documented here:

https://www.gnu.org/prep/standards/html_node/DESTDIR.html

[tprk77]

The Makefile is mostly just a convenience wrapper around Meson, and by extension Ninja. That is to say, depending on your use-case, it may be best to fallback to the underlying commands.

That said, DESTDIR seems to be working for me. Like I can do this:

~/ttfrm$ DESTDIR=/tmp/ttfrm make install
cd build && ninja install
[0/1] Installing files.
Installing /home/tim/ttfrm/ttfrm/tfrm.hpp to /tmp/ttfrm/usr/local/include/ttfrm
Installing /home/tim/ttfrm/ttfrm/tfrm_tree.hpp to /tmp/ttfrm/usr/local/include/ttfrm
Installing /home/tim/ttfrm/build/meson-private/ttfrm.pc to /tmp/ttfrm/usr/local/lib/x86_64-linux-gnu/pkgconfig

On the other hand, prefix is not working through the Makefile, and I think using it will require using Meson directly, such as:

~/ttfrm$ meson --prefix=/prefix build
...
~/ttfrm$ cd build
~/ttfrm/build$ DESTDIR=/tmp/ttfrm ninja install
...
Installing /home/tim/ttfrm/ttfrm/tfrm.hpp to /tmp/ttfrm/prefix/include/ttfrm
Installing /home/tim/ttfrm/ttfrm/tfrm_tree.hpp to /tmp/ttfrm/prefix/include/ttfrm
Installing /home/tim/ttfrm/build/meson-private/ttfrm.pc to /tmp/ttfrm/prefix/lib/x86_64-linux-gnu/pkgconfig