cmake: Fix googletest deprecated warnings by using target_compile_options()

Previously, we attempted to disable deprecated declarations warnings when
building gtest by adding `-Wno-deprecated-declarations` to the COMPILE_OPTIONS
property of the googletest directory. However, this approach failed to apply
the option when actually building gtest.

This change applies the compile option directly to the `gtest` target using
target_compile_options() instead. Verified by forcing the condition to TRUE
and confirming the option is included when building `gtest-all.cc` through
`cmake --build ~/dev/ceph/build --target gtest --verbose`.

Signed-off-by: Kefu Chai <[email protected]>

Author: tchaikov

src/CMakeLists.txt

@@ -777,9 +777,8 @@ if(WITH_TESTS)
       $<TARGET_PROPERTY:gtest,INTERFACE_INCLUDE_DIRECTORIES>)
     if(NOT COMPILER_IGNORES_DEPRECATED_DECL_IN_SYSTEM_HEADERS)
       # See https://github.com/llvm/llvm-project/issues/76515
-      set_property(DIRECTORY googletest
-        APPEND
-        PROPERTY COMPILE_OPTIONS "-Wno-deprecated-declarations")
+      target_compile_options(gtest
+        PRIVATE "-Wno-deprecated-declarations")
     endif()
     add_library(GTest::GTest ALIAS gtest)
     add_library(GTest::Main ALIAS gtest_main)