Use pkg-config to find GoogleTest and Benchmark.

The GNU make CI workflow is currently broken on macOS because the Homebrew
prefix differs between X64 and ARM64. It turns out that the compiler can't
find GoogleTest (and Benchmark) anymore via the default path(s). We should
really have been specifying the path(s) explicitly long before now anyway.

Change-Id: If526389aa89b20b64ccfcc486769050c351e759f
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63030
Reviewed-by: Paul Wankadia <junyer@google.com>
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>

Makefile

@@ -29,6 +29,13 @@ LDABSL=$(shell $(PKG_CONFIG) $(ABSL_DEPS) --libs | sed -e 's/-Wl / /g')
 # CCICU=$(shell $(PKG_CONFIG) icu-uc --cflags) -DRE2_USE_ICU
 # LDICU=$(shell $(PKG_CONFIG) icu-uc --libs)
 
+# 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 $(PKG_CONFIG) gtest --libs-only-L)
+CCBENCHMARK=$(shell $(PKG_CONFIG) benchmark --cflags)
+LDBENCHMARK=$(shell $(PKG_CONFIG) benchmark --libs-only-L)
+
 # To build against PCRE for testing and benchmarking,
 # uncomment the next two lines:
 # CCPCRE=-I/usr/local/include -DUSEPCRE
@@ -39,8 +46,8 @@ CXX?=g++
 CXXFLAGS?=-O3 -g
 LDFLAGS?=
 # required
-RE2_CXXFLAGS?=-pthread -Wall -Wextra -Wno-unused-parameter -Wno-missing-field-initializers -I. $(CCABSL) $(CCICU) $(CCPCRE)
-RE2_LDFLAGS?=-pthread $(LDABSL) $(LDICU) $(LDPCRE)
+RE2_CXXFLAGS?=-pthread -Wall -Wextra -Wno-unused-parameter -Wno-missing-field-initializers -I. $(CCABSL) $(CCICU) $(CCGTEST) $(CCBENCHMARK) $(CCPCRE)
+RE2_LDFLAGS?=-pthread $(LDABSL) $(LDICU) $(LDGTEST) $(LDBENCHMARK) $(LDPCRE)
 AR?=ar
 ARFLAGS?=rsc
 NM?=nm
@@ -233,22 +240,22 @@ obj/so/libre2.$(SOEXT): $(SOFILES) libre2.symbols libre2.symbols.darwin
 .PRECIOUS: obj/dbg/test/%
 obj/dbg/test/%: obj/dbg/libre2.a obj/dbg/re2/testing/%.o $(DTESTOFILES)
 	@mkdir -p obj/dbg/test
-	$(CXX) -o $@ obj/dbg/re2/testing/$*.o $(DTESTOFILES) -lgtest -lgtest_main obj/dbg/libre2.a $(RE2_LDFLAGS) $(LDFLAGS)
+	$(CXX) -o $@ obj/dbg/re2/testing/$*.o $(DTESTOFILES) obj/dbg/libre2.a $(RE2_LDFLAGS) $(LDFLAGS) -lgtest -lgtest_main
 
 .PRECIOUS: obj/test/%
 obj/test/%: obj/libre2.a obj/re2/testing/%.o $(TESTOFILES)
 	@mkdir -p obj/test
-	$(CXX) -o $@ obj/re2/testing/$*.o $(TESTOFILES) -lgtest -lgtest_main obj/libre2.a $(RE2_LDFLAGS) $(LDFLAGS)
+	$(CXX) -o $@ obj/re2/testing/$*.o $(TESTOFILES) obj/libre2.a $(RE2_LDFLAGS) $(LDFLAGS) -lgtest -lgtest_main
 
 # Test the shared lib, falling back to the static lib for private symbols
 .PRECIOUS: obj/so/test/%
 obj/so/test/%: obj/so/libre2.$(SOEXT) obj/libre2.a obj/re2/testing/%.o $(TESTOFILES)
 	@mkdir -p obj/so/test
-	$(CXX) -o $@ obj/re2/testing/$*.o $(TESTOFILES) -lgtest -lgtest_main -Lobj/so -lre2 obj/libre2.a $(RE2_LDFLAGS) $(LDFLAGS)
+	$(CXX) -o $@ obj/re2/testing/$*.o $(TESTOFILES) -Lobj/so -lre2 obj/libre2.a $(RE2_LDFLAGS) $(LDFLAGS) -lgtest -lgtest_main
 
 obj/test/regexp_benchmark: obj/libre2.a obj/re2/testing/regexp_benchmark.o $(TESTOFILES)
 	@mkdir -p obj/test
-	$(CXX) -o $@ obj/re2/testing/regexp_benchmark.o $(TESTOFILES) -lgtest -lbenchmark -lbenchmark_main obj/libre2.a $(RE2_LDFLAGS) $(LDFLAGS)
+	$(CXX) -o $@ obj/re2/testing/regexp_benchmark.o $(TESTOFILES) obj/libre2.a $(RE2_LDFLAGS) $(LDFLAGS) -lgtest -lbenchmark -lbenchmark_main
 
 obj/test/re2_fuzzer: obj/libre2.a obj/re2/fuzzing/re2_fuzzer.o
 	@mkdir -p obj/test