small tweaks to cmake_build

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

Committed: https://skia.googlesource.com/skia/+/de5973b05be6ecc80309cded8541f9c305135d15

[mtklein_C, 2015-09-15 17:01:41]

mtklein@chromium.org changed reviewers:
+ borenet@google.com

[mtklein_C, 2015-09-15 17:03:32]

The CQ bit was checked by mtklein@chromium.org to run a CQ dry run

[borenet, 2015-09-15 17:03:46]

LGTM

[commit-bot: I haz the power, 2015-09-15 17:03:48]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1345663004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1345663004/1

[mtklein_C, 2015-09-15 17:10:03]

The CQ bit was unchecked by mtklein@chromium.org

[mtklein_C, 2015-09-15 17:10:04]

The CQ bit was checked by mtklein@chromium.org

[commit-bot: I haz the power, 2015-09-15 17:10:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1345663004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1345663004/1

[commit-bot: I haz the power, 2015-09-15 17:17:12]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/de5973b05be6ecc80309cded8541f9c305135d15