PKG_CHECK_MODULES() no arg needs to be set to something

Author: tiran

configure

@@ -6394,9 +6394,10 @@ IFS=$as_save_IFS
 dnl libb2 for blake2. _blake2 module falls back to vendored copy.
 AS_VAR_IF([with_builtin_blake2], [yes], [
   PKG_CHECK_MODULES([LIBB2], [libb2], [
+    have_libb2=yes
     AC_DEFINE([HAVE_LIBB2], [1],
               [Define to 1 if you want to build _blake2 module with libb2])
-  ])
+  ], [have_libb2=no])
 ])
 
 # --with-experimental-isolated-subinterpreters