BUILD.gn file for submodules

When submodules are enabled, we read the .git file to find the new .git
location.

Bug: b/253329946
Change-Id: I373e47870b2bd4ecb6638149cefad1bd782635c9
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/818627
Reviewed-by: Craig Stout <cstout@google.com>
1 file changed