diff --git a/configure.ac b/configure.ac
index 40d8c0a..9fa8305 100755
--- a/configure.ac
+++ b/configure.ac
@@ -57,7 +57,11 @@ AS_IF([test "x$with_system_nspr" = "xyes"],
 if test "$OBJ_PATH" != ""; then
 SDK_DIR=$OBJ_PATH/dist
 IDL_DIR=$OBJ_PATH/dist/idl
-ENGINE_CFLAGS="-I$OBJ_PATH/dist/include -I$OBJ_PATH/dist/include/nspr -I$OBJ_PATH/dist/include/mozilla -I$OBJ_PATH/dist/include/dom -I/usr/include/nspr4"
+ENGINE_CFLAGS="-I$OBJ_PATH/dist/include -I$OBJ_PATH/dist/include/mozilla -I$OBJ_PATH/dist/include/dom"
+AS_IF([test "x$with_system_nspr" = "xyes"],
+  [ENGINE_CFLAGS="${ENGINE_CFLAGS} `pkg-config --cflags-only-I nspr`"],
+  [ENGINE_CFLAGS="${ENGINE_CFLAGS} -I$OBJ_PATH/dist/include/nspr"]
+)
 ENGINE_LIBS="-L$OBJ_PATH/dist/sdk/lib -lxpcomglue_s -lxul"
 else
 PKG_CHECK_MODULES(ENGINE, libxul $NSPR,