configure: fix detection of polylib srcdir when --with-polylib-builddir is given
configure: fix detection of polylib srcdir when --with-polylib-builddir is given

Commit f8036e51d55e1aa422b4b744c5ea62de54e2bd5c (configure: replace
--with-polylib-{include,library} by --with-polylib-builddir) introduced
the automatic detection of srcdir of PolyLib given its buildir.
However, the relative srcdir was used, which has no guarantee
of working when applied to a different working directory.
In fact, it will definitely not work when srcdir='.'.
Use abs_srcdir instead.

SaberMod/cloog-current,heysion/cloog-clone,evaautomation/cloog,SaberMod/cloog-current,heysion/cloog-clone,Syllo/cloog,Distrotech/cloog,epowers/cloog,VanirLLVM/toolchain_cloog,evaautomation/cloog,Distrotech/cloog,epowers/cloog,Syllo/cloog,UBERTC/cloog,VanirLLVM/toolchain_cloog,periscop/cloog,cfx-next/toolchain_cloog-upstream,cfx-next/toolchain_cloog-upstream,UBERTC/cloog,Distrotech/cloog,periscop/cloog,UBERTC/cloog,heysion/cloog-clone,cfx-next/toolchain_cloog-upstream,VanirLLVM/toolchain_cloog,epowers/cloog