Add GNU regex bundle from upstream ctags and use it if missing

If we don't have `regcomp()` (e.g. on Windows), use the bundled GNU
regex from upstream ctags.

configure.ac

@@ -44,6 +44,14 @@ AC_CHECK_HEADERS([fcntl.h glob.h stdlib.h sys/time.h errno.h limits.h])
 # Checks for dependencies needed by ctags
 AC_CHECK_HEADERS([fnmatch.h direct.h io.h sys/dir.h])
 AC_DEFINE([HAVE_STDBOOL_H], [1], [whether or not to use <stdbool.h>.])
+AC_CHECK_FUNC([regcomp],
+		[have_regcomp=yes],
+		[have_regcomp=no
+		 dnl various stuff for ctags/gnu_regex/
+		 AC_CHECK_HEADERS([langinfo.h locale.h libintl.h wctype.h wchar.h])
+		 AC_CHECK_FUNCS([memcpy isblank wcrtomb mbrtowc wcscoll])
+		 AC_FUNC_ALLOCA])
+AM_CONDITIONAL([USE_BUNDLED_REGEX], [test "xno" = "x$have_regcomp"])
 
 # Checks for typedefs, structures, and compiler characteristics.
 AC_TYPE_OFF_T

ctags/Makefile.am

@@ -176,3 +176,25 @@ libctags_la_SOURCES = \
 	main/xtag.h \
 	main/xtag_p.h \
 	$(parsers)
+
+# build bundled GNU regex if needed
+if USE_BUNDLED_REGEX
+noinst_LTLIBRARIES += libgnu_regex.la
+libgnu_regex_la_SOURCES = \
+	gnu_regex/regex.c \
+	gnu_regex/regex.h
+# regex.c includes other sources we have to distribute
+EXTRA_libgnu_regex_la_SOURCES = \
+	gnu_regex/regcomp.c \
+	gnu_regex/regex.c \
+	gnu_regex/regex.h \
+	gnu_regex/regex_internal.c \
+	gnu_regex/regex_internal.h \
+	gnu_regex/regexec.c
+libgnu_regex_la_CPPFLAGS = -D__USE_GNU
+EXTRA_DIST = \
+	gnu_regex/README.txt
+
+libctags_la_LIBADD = libgnu_regex.la
+AM_CPPFLAGS += -I$(srcdir)/gnu_regex
+endif

ctags/gnu_regex/README.txt

@@ -0,0 +1,5 @@
+These source files were taken from the GNU glibc-2.10.1 package.
+
+    ftp://ftp.gnu.org/gnu/glibc/glibc-2.10.1.tar.bz2
+
+Minor changes were made to eliminate compiler errors and warnings.