Ask `pkg-config` about `gtest_main` as well.

vcpkg installs `libgtest_main.a` into a `manual-link` subdirectory
for some reason...

Change-Id: I0842281094dae10ee82e72c22c165d3a8141cbe9
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63090
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>