Internal regex.h is always used

[Reuben Thomas]

Even when configure detects a working system regex, or
--without-included-regex is used,

#include "regex.h"

(which is what gnulib tells you to use) uses the included regex.h, not
the system one. There are obvious potential problems with this: if the
ABI changes you could end up with a header that doesn't match the
linked code, and even without that you can end up thinking you have
features (e.g. syntax bits) which are not in fact available in the
code.

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