hi Nobody mentionned one quite easy solution: use ccache It's built-in in makepkg so you have only to remove the ! before ccache in BUILDENV variable in /etc/makepkg.conf and install also ccache of course. Then for any C or C++ program that you compile with makepkg, every file that has already been compiled once is saved. If ever you make a mistake in a PKGBUILD and have to recompile a twice a software, ccache is your saver because the second time is way faster. Just try it. Note, as emacs might be written in something else than C or C++ like some sort of lisp that might not work. or may be , it could work; I don't know emacs. Vim rocks \o/ You might want to clean your ccache cache frome time to times too.
