try_compile: Honor CMAKE_WARN_DEPRECATED in test project

This causes the `-Wno-deprecated` option to be honored even inside a
`try_compile` test project, which is needed to suppress all deprecation
warnings as the option documents.

Closes: #16446

Tests/RunCMake/try_compile/RunCMakeTest.cmake

@@ -18,6 +18,7 @@ run_cmake(NonSourceCompileDefinitions)
 
 set(RunCMake_TEST_OPTIONS --debug-trycompile)
 run_cmake(PlatformVariables)
+run_cmake(WarnDeprecated)
 unset(RunCMake_TEST_OPTIONS)
 
 run_cmake(TargetTypeExe)

Tests/RunCMake/try_compile/WarnDeprecated.cmake

@@ -0,0 +1,19 @@
+enable_language(C)
+
+set(CMAKE_WARN_DEPRECATED SOME_VALUE)
+
+try_compile(result ${CMAKE_CURRENT_BINARY_DIR}
+  SOURCES ${CMAKE_CURRENT_SOURCE_DIR}/src.c
+  OUTPUT_VARIABLE out
+  )
+if(NOT result)
+  message(FATAL_ERROR "try_compile failed:\n${out}")
+endif()
+
+# Check that the cache was populated with our custom variable.
+file(STRINGS ${CMAKE_BINARY_DIR}/CMakeFiles/CMakeTmp/CMakeCache.txt entries
+  REGEX CMAKE_WARN_DEPRECATED:UNINITIALIZED=${CMAKE_WARN_DEPRECATED}
+  )
+if(NOT entries)
+  message(FATAL_ERROR "try_compile did not populate cache as expected")
+endif()