diff --git a/configure.ac b/configure.ac
index cd52d7f..65a47b4 100644
--- a/configure.ac
+++ b/configure.ac
@@ -102,6 +102,7 @@ case $host in
 esac
 
 AX_CHECK_COMPILE_FLAG([-Wall],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wall"],,[[$CXXFLAG_WERROR]])
+AX_CHECK_COMPILE_FLAG([-Wundef], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wundef"], [], [$CXXFLAG_WERROR])
 AX_CHECK_COMPILE_FLAG([-fvisibility=hidden],[CXXFLAGS="$CXXFLAGS -fvisibility=hidden"],[],[$CXXFLAG_WERROR])
 
 if test "x$use_ccache" != "xno"; then
diff --git a/src/int_utils.h b/src/int_utils.h
index 2bad57d..a6b89cd 100644
--- a/src/int_utils.h
+++ b/src/int_utils.h
@@ -159,7 +159,7 @@ static inline int CountBits(I val, int max) {
     }
     if (!ret) return 0;
     return index + 1;
-#elif HAVE_CLZ
+#elif defined(HAVE_CLZ)
     (void)max;
     if (val == 0) return 0;
     if (std::numeric_limits<unsigned>::digits >= std::numeric_limits<I>::digits) {