Add rustc configuration output as WiX variables

[volks73]

Similar to #118, there is the output from the rustc --print cfg command that could be passed to the WiX compiler (candle.exe) as WiX variables. See #126 for an example output and an initial discussion on which output would be useful as WiX variables. Relevant comments reposted here:

roblabla@roblabla-work:~/vm-image$ rustc --print cfg --target i686-pc-windows-msvc
debug_assertions
target_arch="x86"
target_endian="little"
target_env="msvc"
target_family="windows"
target_feature="fxsr"
target_feature="sse"
target_feature="sse2"
target_os="windows"
target_pointer_width="32"
target_vendor="pc"
windows

We should add some of these as WiX Variables, too:

• target_env = TargetEnv
• target_vendor = TargetVendor

I am not sure the rest are needed or useful.

[volks73]

Any listed target_feature might be useful as well, i.e. crt-static (#115). I guess each one would become TargetFeature<NAME>. If set, then it will be defined. If not set or present in the output, then it will not be set. The ifdef preprocessor directive could be used by developers in the WXS files to toggle installer features based on target features.

[volks73]

As of 8448e17, the TargetEnv and TargetVendor WiX variables are available at least.

The TargetFeature<NAME> variables are still a work-in-progress.