Xcode: Quote variable names with invalid characters in them

Allows us to have scoped variables such as eg FOO[arch=armv7].

We could quote all variables, but Xcode doesn't, and we try to stay
close to the native behavior.

Change-Id: Ia6634a33e42031fe7e69c4f680803fa347e5de4a
Reviewed-by: Andy Shaw <andy.shaw@digia.com>