Make Makefile more friendly to packaging

[gdsotirov]

Allow additional CFLAGS and have all the installation paths in variables, so easily overrideable from environment (e.g. with make -e).

[LudovicRousseau]

Using CFLAGS="whatever" make should already use your defined value of CFLAGS. No?

Also I would use BINDIR ?= /bin so that it is easy to overwrite it using: BINDIR=foobar make
https://www.gnu.org/software/make/manual/make.html#index-_003f_003d

Comments?

[LudovicRousseau]

Partly applied in 22b65fe

[gdsotirov]

For the ?= operator no comments at all - it's even better like this. But I used different variable for CFLAGS, because of make -e. I think without it it would be OK, but I'll check it tomorrow and write back.

[gdsotirov]

So now with the ?= operator and without -e flag everything is OK - I'm able to set both CFLAGS and INSTALL_* variables. Thanks for applying the patch. The issue could be closed.