@techee 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
Yes, this works.
Though maybe the "correct" way is to have `__USE_GNU` defined consistently - when it's defined for the `gnu_regex` library, it should be defined also for users of the library otherwise there might be incompatible declarations coming from the header (not sure if it's really the case, seems to work fine).