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>

Makefile

diff --git a/Makefile b/Makefile
index 65b52f6..5d6d6c0 100644
--- a/Makefile
+++ b/Makefile
@@ -31,8 +31,8 @@
 
 # Build against GoogleTest and Benchmark for... testing and benchmarking.
 # Capture only the `-L` flags for now; we will pass the `-l` flags later.
-CCGTEST=$(shell $(PKG_CONFIG) gtest --cflags)
-LDGTEST=$(shell $(PKG_CONFIG) gtest --libs-only-L)
+CCGTEST=$(shell $(PKG_CONFIG) gtest gtest_main --cflags)
+LDGTEST=$(shell $(PKG_CONFIG) gtest gtest_main --libs-only-L)
 CCBENCHMARK=$(shell $(PKG_CONFIG) benchmark --cflags)
 LDBENCHMARK=$(shell $(PKG_CONFIG) benchmark --libs-only-L)