Only use libc++ with clang if asked explicitly
Only use libc++ with clang if asked explicitly

ddemidov/amgcl,huahbo/amgcl,ddemidov/amgcl,huahbo/amgcl,ddemidov/amgcl,dondieselkopf/amgcl,huahbo/amgcl,dondieselkopf/amgcl,dondieselkopf/amgcl,ddemidov/amgcl