Enable stlport thread support.

This fixes a bug when used in multi-threaded environments.

Change-Id: I0218f29a3cc685b2ada6d1ce15331814f7bafc09
diff --git a/stlport/stl/config/_android.h b/stlport/stl/config/_android.h
index 6ad94ba..4480d47 100644
--- a/stlport/stl/config/_android.h
+++ b/stlport/stl/config/_android.h
@@ -27,8 +27,8 @@
 // No pthread_spinlock_t in Android
 #define _STLP_DONT_USE_PTHREAD_SPINLOCK 1
 
-// Do not enable thread support
-#define _NOTHREADS 1
+// Enable thread support
+#undef _NOTHREADS
 
 // Little endian platform.
 #define _STLP_LITTLE_ENDIAN 1