feat: Add -Wmissing-declarations for GCC

As we start to add free standing functions, we should make
sure that they're either `static` or have a declaration in
some (public) header.

cmake/modules/CheckCompiler.cmake

@@ -67,7 +67,7 @@ if (CMAKE_SYSTEM_NAME MATCHES Linux)
 
      # Configure build types
      set(CMAKE_CONFIGURATION_TYPES "RELWITHDEBINFO" "NIGHTLY" "TEST" "RELEASE" "DEBUG" "DEBUGFULL" "PROFILE" "ARRAY_CHECK")
-     set(_FR_COMMON_CXX_FLAGS "-Wshadow -Wall")
+     set(_FR_COMMON_CXX_FLAGS "-Wshadow -Wall -Wmissing-declarations")
 
      set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "${_FR_COMMON_CXX_FLAGS} -O2 -g")
      set(CMAKE_CXX_FLAGS_NIGHTLY        "${_FR_COMMON_CXX_FLAGS} -O0 -g")