cmake: use target aliases from local Google Test checkout. (#1083)

* cmake: use target aliases from local Google Test checkout.

`ABSL_FIND_GOOGLETEST` is a dependent option, it is always `OFF` when
`ABSL_USE_EXTERNAL_GOOGLETEST` is `OFF` (i.e. when Google Test is
checked out locally).

`GTest::` target aliases are to be added only if `find_package(GTest)`
has been actually used.

* cmake: clarify adding GTest:: aliases.

CMakeLists.txt

@@ -134,8 +134,14 @@ if(BUILD_TESTING AND ABSL_BUILD_TESTING)
   if (ABSL_USE_EXTERNAL_GOOGLETEST)
     if (ABSL_FIND_GOOGLETEST)
       find_package(GTest REQUIRED)
-    else()
-      if (NOT TARGET gtest AND NOT TARGET GTest::gtest)
+    elseif(NOT TARGET GTest::gtest)
+      if(TARGET gtest)
+        # When Google Test is included directly rather than through find_package, the aliases are missing.
+        add_library(GTest::gtest ALIAS gtest)
+        add_library(GTest::gtest_main ALIAS gtest_main)
+        add_library(GTest::gmock ALIAS gmock)
+        add_library(GTest::gmock_main ALIAS gmock_main)
+      else()
         message(FATAL_ERROR "ABSL_USE_EXTERNAL_GOOGLETEST is ON and ABSL_FIND_GOOGLETEST is OFF, which means that the top-level project must build the Google Test project. However, the target gtest was not found.")
       endif()
     endif()
@@ -157,14 +163,6 @@ if(BUILD_TESTING AND ABSL_BUILD_TESTING)
     include(CMake/Googletest/DownloadGTest.cmake)
   endif()
 
-  if (NOT ABSL_FIND_GOOGLETEST)
-    # When Google Test is included directly rather than through find_package, the aliases are missing.
-    add_library(GTest::gtest ALIAS gtest)
-    add_library(GTest::gtest_main ALIAS gtest_main)
-    add_library(GTest::gmock ALIAS gmock)
-    add_library(GTest::gmock_main ALIAS gmock_main)
-  endif()
-
   check_target(GTest::gtest)
   check_target(GTest::gtest_main)
   check_target(GTest::gmock)