use _GNU_SOURCE instead of _POSIX_C_SOURCE

CMakeLists.txt update to add compile flags and require pthread

CMakeLists.txt

@@ -1,6 +1,12 @@
 cmake_minimum_required(VERSION 3.0.0)
 project(C-Thread-Pool LANGUAGES C)
 
+# define the DEBUG macro when building in debug mode
+set(CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS_DEBUG} -DTHPOOL_DEBUG -Wall -Wextra")
+
+# require pthreads to be available
+find_package(Threads REQUIRED)
+
 install(FILES thpool.h DESTINATION include/C-Thread-Pool)
 
 add_library(thpool_static STATIC thpool.c)

thpool.c

@@ -8,7 +8,7 @@
  *
  ********************************/
 
-#define _POSIX_C_SOURCE 200809L
+#define _GNU_SOURCE
 #include <unistd.h>
 #include <signal.h>
 #include <stdio.h>