Customize the stack size of the threads used by pupnp through the new THREAD_STACK_SIZE variable

This patch allows a user to customize the stack size of the threads used by
pupnp through the new THREAD_STACK_SIZE variable. This is especially useful
on embedded systems with limited memory where the user can set THREAD_STACK_SIZE
to ITHREAD_STACK_MIN.

However, as this modification can have side effects, I set 0 as the default
value, so threads will continue to use the default stack size of the system
(which varies greatly as stated in
https://computing.llnl.gov/tutorials/pthreads/).
(cherry picked from commit 467f9987a1)

threadutil/inc/ThreadPool.h

@@ -115,6 +115,10 @@ typedef enum priority {
 #define DEFAULT_MAX_THREADS 10
 
 
+/*! default stack size used by TPAttrInit */
+#define DEFAULT_STACK_SIZE 0
+
+
 /*! default jobs per thread used by TPAttrInit */
 #define DEFAULT_JOBS_PER_THREAD 10
 
@@ -182,6 +186,10 @@ typedef struct THREADPOOLATTR
 	/* maxThreads, ThreadPool will never have more than this number of threads */
 	int maxThreads;
 
+	/* stackSize (in bytes), this is the minimum stack size allocated for each
+	 * thread */
+	size_t stackSize;
+
 	/* maxIdleTime (in milliseconds) this is the maximum time a thread will
 	 * remain idle before dying */
 	int maxIdleTime;
@@ -522,6 +530,20 @@ int TPAttrSetMaxThreads(ThreadPoolAttr *attr, int maxThreads);
 int TPAttrSetMinThreads(ThreadPoolAttr *attr, int minThreads);
 
 
+/****************************************************************************
+ * Function: TPAttrSetStackSize
+ *
+ *  Description:
+ *      Sets the stack size for the thread pool attributes.
+ *  Parameters:
+ *      attr - must be valid thread pool attributes.
+ *      stackSize - value to set
+ *  Returns:
+ *      Always returns 0.
+ *****************************************************************************/
+int TPAttrSetStackSize(ThreadPoolAttr *attr, size_t stackSize);
+
+
 /****************************************************************************
  * Function: TPAttrSetIdleTime
  *