Guard against the `BUILDDIR' setting in 'makepkg.conf'

[Alexander-Shukaev]

makepkg.conf has its own BUILDDIR setting. If this one is set to a different value than builddir in apacman.conf, this will cause problems. Give it a shot and see yourself. For instance:

builddir="/tmp/apacman-${UID}/build"
tmpdir="/tmp/apacman-${UID}/tmp"

and

BUILDDIR="/tmp/makepkg-${UID}/build"

To workaround this right now, I have to set some canary variable in apacman.conf, like INSIDE_APACMAN and only set BUILDDIR in makepkg.conf if INSIDE_APACMAN is not set.

#
# CANARY
#

export INSIDE_APACMAN=1