Add option to use an externally provided GoogleTest target (for usage of abseil as add_subdirectory target) (#647)

CMakeLists.txt

@@ -81,6 +81,9 @@ endif()
 ## pthread
 find_package(Threads REQUIRED)
 
+option(ABSL_USE_EXTERNAL_GOOGLETEST
+  "If ON, abseil will assume that the targets for googletest are already provided by the including project folder. This makes sense when abseil is used with add_subproject." OFF)
+
 option(ABSL_USE_GOOGLETEST_HEAD
   "If ON, abseil will download HEAD from googletest at config time." OFF)
 
@@ -99,7 +102,7 @@ endif()
 
 ## check targets
 if(BUILD_TESTING)
-
+  if (NOT ABSL_USE_EXTERNAL_GOOGLETEST)
     set(absl_gtest_build_dir ${CMAKE_BINARY_DIR}/googletest-build)
     if(${ABSL_USE_GOOGLETEST_HEAD})
       set(absl_gtest_src_dir ${CMAKE_BINARY_DIR}/googletest-src)
@@ -107,6 +110,7 @@ if(BUILD_TESTING)
       set(absl_gtest_src_dir ${ABSL_LOCAL_GOOGLETEST_DIR})
     endif()
     include(CMake/Googletest/DownloadGTest.cmake)
+  endif()
 
   check_target(gtest)
   check_target(gtest_main)