configure.ac: only create interface/Makefile if user has clang
configure.ac: only create interface/Makefile if user has clang

This allows users without clang to perform a "make dist".

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

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