Make GDB use libiberty regex implementation.

Make GDB use libiberty regex implementation.
* gdb_regex.h: Normalize protection against multiple inclusion.
Include "xregex.h" instead of "gnu-regex.h".
* cli/cli-cmds.c: Include "gdb_regex.h" instead of "gnu-regex.h".
* cli/cli-decode.c: Likewise.
* Makefile.in (REGEX): Remove.
(GDB_CFLAGS): Remove reference to gnu-regex.h in comment.
(ADD_FILES, ADD_DEPS): Remove $(REGEX).
(POSSLIBS): Remove.
(TAGFILES_NO_SRCDIR): Remove $(POSSLIBS).
(irix5-nat.o, solib.o, solib-svr4.o, source.o, symtab.o,
xcoffsolib.o, cli-decode.o, cli-cmd.o): Replace gnu-regex.h with
gdb_regex.h in list of dependencies.
(gnu-regex.o): Remove rule.
* gnu-regex.c, gnu-regex.h: Remove files.