So, you're saying that all the features in the gnulib version of GNU
regex have been in glibc for sufficiently long that I am very unlikely
ever to use a system with a sufficiently old version of glibc that it
is lacking a feature (e.g. a particular syntax flag) that is present
in GNU regex? Or that in this case, the gnulib tests will identify
this problem and force --with-included-regex? If so, I agree, there is
no real problem here (other than for me: I modified regex.h to add my
RE_PLAIN, and then found that if I tested for RE_PLAIN I could still
end up with the system library; however, this is, I agree, a rare
case).