makepkg.conf: Use all cores by default for MAKEFLAGS

This is a better default in my opinion than using only one CPU core, when nowadays even smartphones often have Octacore CPUs.

If you don't want to change the default, I suggest to comment the line again (but use -j$(nproc) instead of -j2).

Allan McRae demands that Arch Linux should change this setting first. See also: pacman/pacman!178 (closed)