- Add a way to give additional compiler flags with USER_CFLAGS.
- Add possibility to specify an file name extension for generated executables (PROGEXT).
Merge pull request #66 from groessler/something_to_pull

New src/Makefile variables: USER_CFLAGS and PROGEXT
Moved USER_CFLAGS to the other flags.
Merge remote-tracking branch 'upstream/master'