Add --enable-chksum configure flag to require checksum support

... at compile time

It can still fail later, though.

configure.ac

@@ -42,6 +42,12 @@ fi
 AC_SUBST(VMOD_BLOBDIGEST)
 
 AM_CONDITIONAL(CHKSUM, test x$VMOD_BLOB != x && test x$VMOD_BLOBDIGEST != x)
+AC_ARG_ENABLE([chksum], [AS_HELP_STRING([--enable-chksum],
+    [require chksum feature, fail build if vmod_blob/vmod_blobdigest not found])],
+  [if test "x$CHKSUM_TRUE" != x ; then
+	AC_MSG_FAILURE([checksum support required (see above)])
+   fi],
+  [])
 
 AC_CONFIG_FILES([
 	Makefile