build,cmake: add _GNU_SOURCE to CFLAGS

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

CMakeLists.txt

@@ -12,7 +12,7 @@ if(MSVC)
   file(RENAME ${CMAKE_CURRENT_BINARY_DIR}/include/json_config.h.win32 ${CMAKE_CURRENT_BINARY_DIR}/include/json_config.h)
 elseif(UNIX)
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror")
-  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Werror")
+  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Werror -D_GNU_SOURCE")
   execute_process(COMMAND echo ${CMAKE_CURRENT_SOURCE_DIR} WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR})
   execute_process(COMMAND sh autogen.sh WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR})
   execute_process(COMMAND ./configure WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR})