autotools.bbclass: using relative paths for acpaths

autotools.bbclass: using relative paths for acpaths

Fix autotools.bbclass to use relative paths for acpaths instead of
absolute ones. Since absolute paths may cause potential autoreconf
error like:

    Can't exec "/bin/sh": Argument list too long ...

This error occurs while building coreutils with long TMPDIR, because
it has bunch of m4 files need to be expanded.

[YOCTO #2766]

(From OE-Core rev: aa66ef6598c84231577d139ec7be413e73fac2b1)

Signed-off-by: Wenzong Fan <wenzong.fan@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>