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

This will fail at some point during the installation anyway,
so it's better to check early.

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

KangDroidSMProject/ISL,inducer/isl-mirror,evaautomation/isl,evaautomation/isl,Distrotech/isl,pierrotdelalune/isl,serge-sans-paille/isl,SaberMod/isl-current,simbuerg/isl,Meinersbur/isl,simbuerg/isl,PollyLabs/isl,Distrotech/isl,KangDroidSMProject/ISL,PollyLabs/isl,nicolasvasilache/isl,BenzoSM/isl,epowers/isl,tobig/isl,BobSaget-Mod/libisl,KangDroidSMProject/ISL,PollyLabs/isl,simbuerg/isl,cfx-next/toolchain_isl-upstream,abduld/isl,pierrotdelalune/isl,Distrotech/isl,simbuerg/isl,epowers/isl,inducer/isl-mirror,PollyLabs/isl,serge-sans-paille/isl,KangDroidSMProject/ISL,BenzoSM/isl,jleben/isl,VanirLLVM/toolchain_isl,epowers/isl,UBERTC/isl,epowers/isl,SaberMod/isl-current,BenzoSM/isl,BobSaget-Mod/libisl,cfx-next/toolchain_isl-upstream,VanirLLVM/toolchain_isl,inducer/isl-mirror,crossbuild/isl,SaberMod/isl-current,nicolasvasilache/isl,abduld/isl,cfx-next/toolchain_isl-upstream,abduld/isl,crossbuild/isl,cfx-next/toolchain_isl-upstream,UBERTC/isl,tobig/isl,evaautomation/isl,cfx-next/toolchain_isl-upstream,tobig/isl,UBERTC/isl,nicolasvasilache/isl,KangDroidSMProject/ISL,BobSaget-Mod/libisl,serge-sans-paille/isl,BenzoSM/isl,serge-sans-paille/isl,evaautomation/isl,jleben/isl,VanirLLVM/toolchain_isl,epowers/isl,tobig/isl,jleben/isl,crossbuild/isl,Meinersbur/isl,pierrotdelalune/isl,jleben/isl,pierrotdelalune/isl,BobSaget-Mod/libisl,simbuerg/isl,nicolasvasilache/isl,UBERTC/isl,SaberMod/isl-current,Distrotech/isl,pierrotdelalune/isl,Meinersbur/isl,PollyLabs/isl,nicolasvasilache/isl,BenzoSM/isl,Meinersbur/isl,Distrotech/isl,crossbuild/isl,abduld/isl,BobSaget-Mod/libisl,inducer/isl-mirror,inducer/isl-mirror,VanirLLVM/toolchain_isl,serge-sans-paille/isl,jleben/isl,VanirLLVM/toolchain_isl