@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 a bit worried by this, esp. when I see stuff like https://gcc.gnu.org/legacy-ml/fortran/2005-10/msg00365.html (yes it's old, but `__USE_GNU` doesn't seem trusty either).
What is the problem if only defining when building the `gnu_regex` library exactly? I admittedly didn't test, but I'm worried that in can have very unexpected impact on many system headers, potentially even breaking them (as IIUC this is something that should usually not appear if `_GNU_SOURCE` is not defined), so I'd be overly cautious with this. And I don't see what *should* be a problem: *regex.h* seem to check if it's defined or not properly, so not having it seems fine, as what it guards also seems to only be used internally.
Anyway, color me worried, so I'd really like to have all the details :-)