maint: avoid empty regex in syntax checker

maint: avoid empty regex in syntax checker

We were defining 'func_or' as '|VIR_ERROR|...', which when put
inside 'func_re' resulted in a regex that matches everything in
isolation.  Thankfully, we always used func_re with a leading
anchor \<, and since the empty regex does not start a word, we
happened to get the result we wanted; but it's better to define
func_or without a leading space converted into a leading empty
alternation.

* cfg.mk (func_or): Strip leading space.