Add missing include for ffsl() test.
diff --git a/configure.ac b/configure.ac
index e704d2d..8e6a309 100644
--- a/configure.ac
+++ b/configure.ac
@@ -874,6 +874,7 @@
 dnl platforms that jemalloc currently has a chance of functioning on without
 dnl modification.
 JE_COMPILABLE([a program using ffsl], [
+#include <strings.h>
 #include <string.h>
 ], [
 	{