Introduced callback to Pthread, Win32 and OpenMP backend

driver/others/blas_server_callback.c

@@ -0,0 +1,12 @@
+#include "common.h"
+
+/* global variable to change threading backend from openblas-managed to caller-managed */
+openblas_threads_callback openblas_threads_callback_ = 0;
+
+/* non-threadsafe function should be called before any other
+   openblas function to change how threads are managed */
+   
+void openblas_set_threads_callback_function(openblas_threads_callback callback)
+{
+  openblas_threads_callback_ = callback;
+}