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

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 ""

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@27676 91177308-0d34-0410-b5e6-96231b3b80d8