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.
Update README steps for make

Including the other couple of steps to make for others for whom it is not obvious. Generally people find out about GLEW when using it as dependency of another lib, and they may or may not be familiar with the make process. Leaving it vague leads to more people reporting issues, which I'm aiming to avoid here!
Merge branch 'master' of https://github.com/hasufell/glew
Merge branch 'master' of https://github.com/netpoetica/glew