autoconf/configure.in: don't allow installation in source or build directory
autoconf/configure.in: don't allow installation in source or build directory

It doesn't really make sense to try to install in the source
directory, but people seem to try anyway.
Installation itself fails if you try, so better detect this situation
early.

periscop/piplib,Ced/piplib,Ced/piplib,periscop/piplib,Ced/piplib,periscop/piplib