[CMake] Use modern target_link_library syntax for exectuables.

Author: hageboeck

cmake/modules/RootMacros.cmake

@@ -1294,7 +1294,7 @@ function(ROOT_EXECUTABLE executable)
     set(exe_srcs ${exe_srcs} ${ROOT_RC_SCRIPT})
   endif()
   add_executable(${executable} ${_all} ${exe_srcs})
-  target_link_libraries(${executable} ${ARG_LIBRARIES})
+  target_link_libraries(${executable} PRIVATE ${ARG_LIBRARIES})
 
   if(WIN32 AND ${executable} MATCHES \\.exe)
     set_target_properties(${executable} PROPERTIES SUFFIX "")
@@ -1659,7 +1659,7 @@ 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} gtest gtest_main gmock gmock_main ROOTUnitTestSupport)
+  target_link_libraries(${test_suite} PRIVATE gtest gtest_main gmock gmock_main ROOTUnitTestSupport)
   target_include_directories(${test_suite} PRIVATE ${CMAKE_CURRENT_BINARY_DIR})
   if (ARG_INCLUDE_DIRS)
     target_include_directories(${test_suite} PRIVATE ${ARG_INCLUDE_DIRS})