Use Gnulib's `regex' module.

This should help with regex portability, as reported in
<http://bugs.gnu.org/10684> for Darwin 8.11.

* m4/gnulib-cache.m4 (gl_MODULES): Add `regex'.

* configure.ac: Remove header checks for regex.h, rxposix.h, and
  rx/rxposix.h.  Remove check for the `regcomp' function.  Remove
  definition of `HAVE_REGCOMP'.  Define `ENABLE_REGEX'.

* libguile/init.c: Check for `ENABLE_REGEX' instead of `HAVE_REGCOMP'.

* libguile/regex-posix.c: Always include <regex.h>.  Remove #ifdefs for
  rxposix.h and co.

lib/strings.in.h

@@ -0,0 +1,123 @@
+/* A substitute <strings.h>.
+
+   Copyright (C) 2007-2012 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU Lesser General Public License as published by
+   the Free Software Foundation; either version 2, or (at your option)
+   any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public License
+   along with this program; if not, write to the Free Software Foundation,
+   Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
+
+#ifndef _@GUARD_PREFIX@_STRINGS_H
+
+#if __GNUC__ >= 3
+@PRAGMA_SYSTEM_HEADER@
+#endif
+@PRAGMA_COLUMNS@
+
+/* Minix 3.1.8 has a bug: <sys/types.h> must be included before <strings.h>.
+   But avoid namespace pollution on glibc systems.  */
+#if defined __minix && !defined __GLIBC__
+# include <sys/types.h>
+#endif
+
+/* The include_next requires a split double-inclusion guard.  */
+#if @HAVE_STRINGS_H@
+# @INCLUDE_NEXT@ @NEXT_STRINGS_H@
+#endif
+
+#ifndef _@GUARD_PREFIX@_STRINGS_H
+#define _@GUARD_PREFIX@_STRINGS_H
+
+#if ! @HAVE_DECL_STRNCASECMP@
+/* Get size_t.  */
+# include <stddef.h>
+#endif
+
+
+/* The definitions of _GL_FUNCDECL_RPL etc. are copied here.  */
+
+/* The definition of _GL_ARG_NONNULL is copied here.  */
+
+/* The definition of _GL_WARN_ON_USE is copied here.  */
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+
+  /* Find the index of the least-significant set bit.  */
+#if @GNULIB_FFS@
+# if !@HAVE_FFS@
+_GL_FUNCDECL_SYS (ffs, int, (int i));
+# endif
+_GL_CXXALIAS_SYS (ffs, int, (int i));
+_GL_CXXALIASWARN (ffs);
+#elif defined GNULIB_POSIXCHECK
+# undef ffs
+# if HAVE_RAW_DECL_FFS
+_GL_WARN_ON_USE (ffs, "ffs is not portable - use the ffs module");
+# endif
+#endif
+
+/* Compare strings S1 and S2, ignoring case, returning less than, equal to or
+   greater than zero if S1 is lexicographically less than, equal to or greater
+   than S2.
+   Note: This function does not work in multibyte locales.  */
+#if ! @HAVE_STRCASECMP@
+extern int strcasecmp (char const *s1, char const *s2)
+     _GL_ARG_NONNULL ((1, 2));
+#endif
+#if defined GNULIB_POSIXCHECK
+/* strcasecmp() does not work with multibyte strings:
+   POSIX says that it operates on "strings", and "string" in POSIX is defined
+   as a sequence of bytes, not of characters.   */
+# undef strcasecmp
+# if HAVE_RAW_DECL_STRCASECMP
+_GL_WARN_ON_USE (strcasecmp, "strcasecmp cannot work correctly on character "
+                 "strings in multibyte locales - "
+                 "use mbscasecmp if you care about "
+                 "internationalization, or use c_strcasecmp , "
+                 "gnulib module c-strcase) if you want a locale "
+                 "independent function");
+# endif
+#endif
+
+/* Compare no more than N bytes of strings S1 and S2, ignoring case,
+   returning less than, equal to or greater than zero if S1 is
+   lexicographically less than, equal to or greater than S2.
+   Note: This function cannot work correctly in multibyte locales.  */
+#if ! @HAVE_DECL_STRNCASECMP@
+extern int strncasecmp (char const *s1, char const *s2, size_t n)
+     _GL_ARG_NONNULL ((1, 2));
+#endif
+#if defined GNULIB_POSIXCHECK
+/* strncasecmp() does not work with multibyte strings:
+   POSIX says that it operates on "strings", and "string" in POSIX is defined
+   as a sequence of bytes, not of characters.  */
+# undef strncasecmp
+# if HAVE_RAW_DECL_STRNCASECMP
+_GL_WARN_ON_USE (strncasecmp, "strncasecmp cannot work correctly on character "
+                 "strings in multibyte locales - "
+                 "use mbsncasecmp or mbspcasecmp if you care about "
+                 "internationalization, or use c_strncasecmp , "
+                 "gnulib module c-strcase) if you want a locale "
+                 "independent function");
+# endif
+#endif
+
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _@GUARD_PREFIX@_STRING_H */
+#endif /* _@GUARD_PREFIX@_STRING_H */