[PATCH] scripts: Do not prepend the prefix to absolute paths

The INSTALL_* variables can be absolute or relative to the prefix path. The current code only handles the case where they are relative. With the patch, the old behaviour is used as long as they are relative, but it fixes the case when they are absolute.

This is important for Exherbo since we have to use absolute paths because of our file system layout. Exherbo has /usr/share for arch-independent files and /usr/$ARCH/

for arch-dependent files, so that files for multiple architectures can be installed in parallel.