compiler_setup.cmake: specify -Wno-error=deprecated-declarations to avoid breaking the build over usage of deprecated things

Author: dconeybe

cmake/compiler_setup.cmake

@@ -86,6 +86,13 @@ if(CXX_CLANG OR CXX_GNU)
   if(FIREBASE_CXX_COMPILER_FLAG_REDUNDANT_MOVE_SUPPORTED)
     list(APPEND common_flags -Wno-error=redundant-move)
   endif()
+
+  # Disable treating "deprecated-declarations" as an error since it's not really a problem
+  # worth breaking the build over.
+  CHECK_CXX_COMPILER_FLAG("-Wno-error=deprecated-declarations" FIREBASE_CXX_COMPILER_FLAG_DEPRECATED_DECLARATIONS_SUPPORTED)
+  if(FIREBASE_CXX_COMPILER_FLAG_DEPRECATED_DECLARATIONS_SUPPORTED)
+    list(APPEND common_flags -Wno-error=deprecated-declarations)
+  endif()
 endif()
 
 if(APPLE)