.emacs: remove git-commit-mode cruft
.emacs: enable flyspell-prog-mode for prog-mode

prog-mode is the mode that all programming language modes inherit from
(new in Emacs 24.1).
.zshrc: add a git-make helper

I don't mind executing 'make -j 8' everytime, but setting CFLAGS by
hand is too much of a pain.