Added support of packaged GTest for running unit tests. (#1226)

Signed-off-by: Vitaly Zaitsev <vitaly@easycoding.org>

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index d32dbd8..f791801 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -34,6 +34,7 @@
 # This option can be used to disable building and running unit tests which depend on gtest
 # in cases where it is not possible to build or find a valid version of gtest.
 option(BENCHMARK_ENABLE_GTEST_TESTS "Enable building the unit tests which depend on gtest" ON)
+option(BENCHMARK_USE_BUNDLED_GTEST "Use bundled GoogleTest. If disabled, the find_package(GTest) will be used." ON)
 
 option(BENCHMARK_ENABLE_LIBPFM "Enable performance counters provided by libpfm" OFF)
 
@@ -308,7 +309,15 @@
   if (BENCHMARK_ENABLE_GTEST_TESTS AND
       NOT (TARGET gtest AND TARGET gtest_main AND
            TARGET gmock AND TARGET gmock_main))
-    include(GoogleTest)
+    if (BENCHMARK_USE_BUNDLED_GTEST)
+      include(GoogleTest)
+    else()
+      find_package(GTest CONFIG REQUIRED)
+      add_library(gtest ALIAS GTest::gtest)
+      add_library(gtest_main ALIAS GTest::gtest_main)
+      add_library(gmock ALIAS GTest::gmock)
+      add_library(gmock_main ALIAS GTest::gmock_main)
+    endif()
   endif()
   add_subdirectory(test)
 endif()