More strict compiler warnings.

* Error when function is missing return type (not just warning).
 * Warn when functions in source don't have corresponding declarations
   in header. In case that it is not an error they should be put in
   anonymous namespace.

src/CMakeLists.txt

@@ -1,4 +1,4 @@
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -Wold-style-cast -Winit-self -pedantic -std=c++0x -fvisibility=hidden")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -Wold-style-cast -Winit-self -Werror=return-type -Wmissing-declarations -pedantic -std=c++0x -fvisibility=hidden")
 
 if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wdouble-promotion")