diff --git a/config/mt-linux b/config/mt-linux
index e3f15eaa8f55..a09e6f18af57 100644
--- a/config/mt-linux
+++ b/config/mt-linux
@@ -1,2 +1,2 @@
 # When using glibc 2 on Linux we must always use vtable thunks.
-CXXFLAGS_FOR_TARGET = $(CXXFLAGS) -fvtable-thunks
+CXXFLAGS_FOR_TARGET = $(CXXFLAGS) -fvtable-thunks -D_GNU_SOURCE