make third_party() static libraries

It's sometimes nice to have a libfoo.a to work with.

I've had to borrow a trick from Chromium's component template [1] to
avoid trying to make static libraries for header or config only targets.

[1] https://cs.chromium.org/chromium/src/build/config/BUILDCONFIG.gn?rcl=ebf3d7e5827761e1f540cc788bbe301e0f6cc674&l=596

Change-Id: I9ec46089c7a4d0a5194ef8ac262dadac10508786
Reviewed-on: https://skia-review.googlesource.com/c/188024
Commit-Queue: Mike Klein <mtklein@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
1 file changed