Makefile improvements: DESTDIR, PREFIX, CXX, all target, .PHONY

[ryandesign]

This pull request makes a number of improvements to the makefile, which MacPorts has been using for over four years:

• Introduces a PREFIX variable so that the user can indicate where it should be installed
• Introduced a DESTDIR variable so that the installation can be staged to another directory first
• Introduces an all target, for clarity
• Declares all non-file targets to be .PHONY
• Makes use of the CXX variable so that the user can change the compiler

[auriamg]

Merged, thanks