[CMake] Move to CMake's native FindGTest

- Remove own FindGTest, use the one from CMake.
  Starting from CMake 3.23, the GTest libraries have canonical names.
- Replace uses of legacy targets like "gtest" in ROOT with canonical
  target names such as GTest::gtest or GTest::gtest_main.
- Create ALIAS targets like in CMake 3.23 such as GTest::gtest_main.
  For CMake < 3.23, this will allow using the standard FindGTest with
  the modern names already in cmake 3.20.

Author: hageboeck

cmake/modules/FindGTest.cmake

@@ -1794,13 +1794,10 @@ function(ROOT_ADD_GTEST test_suite)
   # against. For example, tests in Core should link only against libCore. This could be tricky
   # to implement because some ROOT components create more than one library.
   ROOT_EXECUTABLE(${test_suite} ${source_files} LIBRARIES ${ARG_LIBRARIES})
-  target_link_libraries(${test_suite} PRIVATE gtest gtest_main gmock gmock_main)
+  target_link_libraries(${test_suite} PRIVATE GTest::gtest GTest::gmock GTest::gtest_main GTest::gmock_main)
   if(TARGET ROOT::TestSupport)
     target_link_libraries(${test_suite} PRIVATE ROOT::TestSupport)
   else()
-    # Since we don't inherit the linkage against gtest from ROOT::TestSupport,
-    # we need to link against gtest here.
-    target_link_libraries(${test_suite} gtest)
     message(WARNING "ROOT_ADD_GTEST(${test_suite} ...): The target ROOT::TestSupport is missing. It looks like the test is declared against a ROOT build that is configured with -Dtesting=OFF.
             If this test sends warning or error messages, this will go unnoticed.")
   endif()

cmake/modules/RootMacros.cmake

@@ -1985,6 +1985,15 @@ if (builtin_gtest)
 
 endif()
 
+# Starting from cmake 3.23, the GTest targets will have stable names.
+# ROOT was updated to use those, but for older CMake versions, we have to declare the aliases:
+foreach(LIBNAME gtest_main gmock_main gtest gmock)
+  if(NOT TARGET GTest::${LIBNAME} AND TARGET ${LIBNAME})
+    add_library(GTest::${LIBNAME} ALIAS ${LIBNAME})
+  endif()
+endforeach()
+
+#------------------------------------------------------------------------------------
 if(webgui AND NOT builtin_openui5)
   ROOT_CHECK_CONNECTION("builtin_openui5=ON")
   if(NO_CONNECTION)

cmake/modules/SearchInstalledSoftware.cmake

@@ -16,7 +16,7 @@ target_include_directories(${libname} PUBLIC
   $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/inc/>
   $<INSTALL_INTERFACE:./>
   )
-target_link_libraries(${libname} PUBLIC Core gtest)
+target_link_libraries(${libname} PRIVATE Core GTest::gtest)
 
 # Installation of header and library:
 set_target_properties(${libname} PROPERTIES PUBLIC_HEADER inc/${header_dir}/TestSupport.hxx)

core/testsupport/CMakeLists.txt

@@ -10,7 +10,7 @@ if(CMAKE_CXX_COMPILER_ID STREQUAL Intel)
 endif()
 
 add_library(VectorisedPDFTests STATIC VectorisedPDFTests.cxx)
-target_link_libraries(VectorisedPDFTests PUBLIC gtest ROOT::Gpad ROOT::RooFitCore ROOT::RooFit)
+target_link_libraries(VectorisedPDFTests PUBLIC ROOT::Gpad ROOT::RooFitCore ROOT::RooFit GTest::gtest)
 
 ROOT_ADD_GTEST(testCompatMode testCompatMode.cxx
               LIBRARIES VectorisedPDFTests)