"vis/text-objects/lexer" fails when lpeg is unavailable

[lanodan]

As seen in https://bugs.gentoo.org/768558 when lpeg is unavailable yet lua is present there is at least "vis/text-objects/lexer" failing, as lua is detected and skipped when unavailable I guess you could also skip the ones depending on lpeg.

(I've put lpeg as an optional runtime dependency to be installed as an extra rather than one that gets installed when the lua option is picked)