Fixed build errors when PJ_OS_HAS_CHECK_STACK or PJ_HAS_TCP is disabled

git-svn-id: https://svn.pjsip.org/repos/pjproject/trunk@153 74dad513-b988-da41-8d7b-12977e46ad98
diff --git a/pjlib/include/pj/compat/os_linux.h b/pjlib/include/pj/compat/os_linux.h
index ab26852..24faa22 100644
--- a/pjlib/include/pj/compat/os_linux.h
+++ b/pjlib/include/pj/compat/os_linux.h
@@ -76,7 +76,9 @@
 
 #define PJ_HAS_HIGH_RES_TIMER	    1
 #define PJ_HAS_MALLOC               1
-#define PJ_OS_HAS_CHECK_STACK	    0
+#ifndef PJ_OS_HAS_CHECK_STACK
+#   define PJ_OS_HAS_CHECK_STACK    0
+#endif
 #define PJ_NATIVE_STRING_IS_UNICODE 0
 
 #define PJ_ATOMIC_VALUE_TYPE	    long