compilers: add logging to obscure compiler defines scraper

If this command fails, for example when CXX is something not generic
enough to be a valid universal compiler command (clang -std=c++11
perhaps), we end up with two problems:

- it's impossible to figure out what Meson ran to get that error

- the error report isn't clear on what is stdout and what is stderr, or
  even that that is what the message is about.

  ```
  meson.build:1:0: ERROR: Unable to get clang pre-processor defines:
  error: invalid argument '-std=c++11' not allowed with 'C'
  ```

  What's C doing there and why is Meson talking about it? Answer: that's
  compiler stdout. Say so.

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