Occasionally, a package will use the $srcdir environment variable for
its own purposes and change its behavior during build if it is set.
This is not normally a problem since $srcdir is not normally exported by
abuild. However, if $srcdir is inherited from the environment in order
to override its default value of $startdir/src, it will continue to be
inherited and thus confuse those packages. Instead, check for
$ABUILD_SRCDIR. Likewise for $pkgbasedir and $ABUILD_PKGBASEDIR for
consistency sake.