Fix `make install` to use DESTDIR and follow common conventions (#2673)
Fix `make install` to use DESTDIR and follow common conventions (#2673)

* Use $(DESTDIR) in Makefile install

This is essential for packaging in (not just) Linux distributions.

* Add common install variables bindir, includedir, libdir to Makefile

To adhere widely adopted GNU convention:
https://www.gnu.org/prep/standards/html_node/Directory-Variables.html.

* Rename $(destdir) to $(ponydir) in Makefile

To avoid confusion with standard DESTDIR variable (used in most projects).

sgebbie/ponyc,Theodus/ponyc,CausalityLtd/ponyc,cquinn/ponyc,sgebbie/ponyc,Theodus/ponyc,jemc/ponyc,sgebbie/ponyc,Theodus/ponyc,kulibali/ponyc,cquinn/ponyc,CausalityLtd/ponyc,jemc/ponyc,CausalityLtd/ponyc,jemc/ponyc,kulibali/ponyc,ponylang/ponyc,dipinhora/ponyc,cquinn/ponyc,dipinhora/ponyc,kulibali/ponyc,dipinhora/ponyc,cquinn/ponyc,ponylang/ponyc,sgebbie/ponyc,Theodus/ponyc,sgebbie/ponyc,Theodus/ponyc,ponylang/ponyc,kulibali/ponyc