[arch-general] Set BUILDDIR in makepkg.conf

[Techlive Zheng]

Hey, guys, is there a way to set BUILDDIR in makepkg.conf to a place
like /var/abs/mybuilds/$pkgname?

I tried set BUILDDIR="/var/abs/mybuilds/$pkgname",but it didn't
work,it builds in /var/abs/mybuilds,not in /var/abs/mybuilds/$pkgname.