Use quotes properly so that the possibility of a null variable set is

eliminated. This can happen, for example, if LLVM is configured without
llvm-gcc in which case things like LLVMGCC_VERSION will be empty. In
such cases, deja-gnu fails with:
can't read "llvmgcc_version": no such variable
because it sees:
set llvmgcc_version
instead of:
set llvmgcc_version ""

llvm-svn: 27676