Enable extra compiler checks

Let's enforce additional automatic checks enforced by the compiler in
order to catch possible errors during compilation.

Reviewed-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Petr Štetiar <ynezz@true.cz>

CMakeLists.txt

@@ -4,7 +4,7 @@ PROJECT(umbim C)
 
 OPTION(PROXY OFF)
 
-ADD_DEFINITIONS(-Os -ggdb -Wall -Werror --std=gnu99 -Wmissing-declarations)
+ADD_DEFINITIONS(-Os -ggdb -Wextra -Wall -Werror --std=gnu99 -Wmissing-declarations -Wno-unused-parameter)
 
 SET(CMAKE_SHARED_LIBRARY_LINK_C_FLAGS "")