gtest: support an external gtest

tests/CMakeLists.txt

@@ -32,8 +32,25 @@ add_definitions(-DSTATICLIB)
 # miniupnp changed their static define
 add_definitions(-DMINIUPNP_STATICLIB)
 
-add_subdirectory(gtest)
-include_directories(${gtest_SOURCE_DIR}/include ${gtest_SOURCE_DIR})
+find_package(GTest)
+
+if (GTest_FOUND)
+  include_directories(SYSTEM ${GTEST_INCLUDE_DIRS})
+else ()
+  add_subdirectory(gtest)
+  include_directories(SYSTEM "${gtest_SOURCE_DIR}/include" "${gtest_SOURCE_DIR}")
+
+  # Emulate the FindGTest module's variable.
+  set(GTEST_MAIN_LIBRARIES gtest_main)
+
+  # Ignore some warnings when building gtest binaries.
+  if(NOT MSVC)
+    set_property(TARGET gtest gtest_main
+      APPEND_STRING
+      PROPERTY
+        COMPILE_FLAGS " -Wno-undef -Wno-sign-compare")
+  endif()
+endif ()
 
 add_subdirectory(core_tests)
 add_subdirectory(crypto)
@@ -66,9 +83,5 @@ add_test(
   NAME    hash-target
   COMMAND hash-target-tests)
 
-if(NOT MSVC)
-  set_property(TARGET gtest gtest_main APPEND_STRING PROPERTY COMPILE_FLAGS " -Wno-undef -Wno-sign-compare")
-endif()
-
 add_custom_target(tests DEPENDS coretests difficulty hash performance_tests core_proxy unit_tests)
 set_property(TARGET gtest gtest_main hash-target-tests tests PROPERTY FOLDER "tests")