Add possibility of not compiling OGA files

The plan is to be able to compile and test NGA-only, OGA-only and both

Bug: skia:11837
Change-Id: Ib79cc2c2c437c72def8649392b345648f49300fc
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/416799
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
7 files changed