Made httplib_pthread_self global

doc/api/httplib_pthread_self.md

@@ -0,0 +1,19 @@
+# LibHTTP API Reference
+
+### `httplib_pthread_self();`
+
+### Parameters
+
+*none*
+
+### Return Value
+
+| Type | Description |
+| :--- | :--- |
+|`pthread_t`|An identifier specific for the current thread|
+
+### Description
+
+The function `httplib_pthread_self()` is a platform indepent function which returns an identifier which identifies the current thread. On systems which support it this is done with a call to the `pthread_self()` function. On other systems an OS specific function is used with equivalent functionality.
+
+### See Also