Upgrade warning flags on non-MSVC compilers.

We go from

    -Wall

to

    -Wall -Wextra -Wno-unused-parameter -Werror=format-security -
Wdeclaration-after-statement -std=gnu89.

Author: obilaniu

CMakeLists.txt

@@ -9,7 +9,7 @@ set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/CMakeModules/")
 if(MSVC)
     set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -W3")
 else()
-    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall")
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -Wextra -Wno-unused-parameter -Werror=format-security -Wdeclaration-after-statement -std=gnu89")
 endif()
 
 enable_testing()