CMake: generate SkUserConfig.h with configure_file

  1)  It's simpler.
  2)  It should cache correctly (and not trigger total rebuilds everytime we re-run CMake.)

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1544493002
CQ_EXTRA_TRYBOTS=client.skia.compile:Build-Ubuntu-GCC-x86_64-Release-CMake-Trybot,Build-Mac10.9-Clang-x86_64-Release-CMake-Trybot

Review URL: https://codereview.chromium.org/1544493002
2 files changed