@b4n commented on this pull request.
EXTRA_DIST = \
gnu_regex/README.txt
libctags_la_LIBADD += libgnu_regex.la -AM_CPPFLAGS += -I$(srcdir)/gnu_regex +AM_CPPFLAGS += -I$(srcdir)/gnu_regex -D__USE_GNU
I'm not sure `__USE_GNU` is really necessary in the interface, it seems to guard flags and private members, possibly only actually useful when actually building glibc/gnulib. It guards the `RE_*` macros, but we have no use of them outside gnu_regex itself, it hides some members with a prefix, and some clear GNU extensions we don't need. Nothing looks like it changes behavior depending on the flag, just whether some symbols are declared or not.
So I'd prefer applying this patch to avoid unexpected effects of setting it in other places.