small tweaks to cmake_build

  - streamline how we define $here
  - only make `cmake` when bootstrapping; it's a little less to build

BUG=skia:4269
CQ_EXTRA_TRYBOTS=client.skia.compile:Build-Mac10.9-Clang-x86_64-Release-CMake-Trybot,Build-Ubuntu-GCC-x86_64-Release-CMake-Trybot

Review URL: https://codereview.chromium.org/1345663004
1 file changed