diff --git a/cmake/platforms/unix.cmake b/cmake/platforms/unix.cmake
index 65fa6a3..ca4be72 100644
--- a/cmake/platforms/unix.cmake
+++ b/cmake/platforms/unix.cmake
@@ -58,10 +58,14 @@ if(STRICT AND (CMAKE_C_COMPILER_ID MATCHES "GNU" OR
 endif()
 
 if(STRICT AND (CMAKE_C_COMPILER_ID MATCHES "GNU"))
-  # -Wmissing-declarations means completely different things in GCC and Clang.
+  # -Wmissing-declarations is spelled differently in Clang.
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wmissing-declarations")
 endif()
 
+if(STRICT AND (CMAKE_C_COMPILER_ID MATCHES "Clang"))
+  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wmissing-prototypes")
+endif()
+
 add_compile_definitions(HELP_DIR="${CMAKE_INSTALL_PREFIX}/share/sgt-puzzles/help")
 
 function(get_platform_puzzle_extra_source_files OUTVAR NAME)