--- configure
+++ configure
@@ -2522,6 +2522,7 @@
 ## Main body of script. ##
 ## -------------------- ##
 
+CFLAGS="$CFLAGS -std=gnu99" # needed for ULLONG_MAX
 ac_ext=c
 ac_cpp='$CPP $CPPFLAGS'
 ac_compile='$CC -c $CFLAGS $CPPFLAGS conftest.$ac_ext >&5'