CMakeLists.txt: use external gtest, fallback to fetching the source (#288)

See: #287

Author: barracuda156

CMakeLists.txt

@@ -96,12 +96,17 @@ endif()
 if (PHMAP_BUILD_TESTS)
 
     if (NOT PHMAP_GTEST_LIBS)
-        include(cmake/DownloadGTest.cmake)
-
-        check_target(gtest)
-        check_target(gtest_main)
-        check_target(gmock)
-        set(PHMAP_GTEST_LIBS gmock_main)
+        find_package(GTest CONFIG)
+        if (GTest_FOUND)
+            set(PHMAP_GTEST_LIBS GTest::gmock_main)
+        else()
+            include(cmake/DownloadGTest.cmake)
+
+            check_target(gtest)
+            check_target(gtest_main)
+            check_target(gmock)
+            set(PHMAP_GTEST_LIBS gmock_main)
+        endif()
     endif()
 
     enable_testing()