When building with clang 4.0.0, virsh build fails like this:

gmake[3]: Entering directory '/usr/home/novel/code/libvirt/tools'
  CC       virsh-virsh.o
In file included from virsh.c:45:
In file included from /usr/local/include/readline/readline.h:31:
/usr/local/include/readline/rltypedefs.h:35:22: error: this function declaration is not a prototype [-Werror,-Wstrict-prototypes]
typedef int Function () __attribute__ ((deprecated));
                     ^
                      void
/usr/local/include/readline/rltypedefs.h:36:24: error: this function declaration is not a prototype [-Werror,-Wstrict-prototypes]
typedef void VFunction () __attribute__ ((deprecated));
                       ^
                        void
/usr/local/include/readline/rltypedefs.h:37:26: error: this function declaration is not a prototype [-Werror,-Wstrict-prototypes]
typedef char *CPFunction () __attribute__ ((deprecated));
                         ^
                          void
/usr/local/include/readline/rltypedefs.h:38:28: error: this function declaration is not a prototype [-Werror,-Wstrict-prototypes]
typedef char **CPPFunction () __attribute__ ((deprecated));
                           ^
                            void
In file included from virsh.c:45:
/usr/local/include/readline/readline.h:385:23: error: this function declaration is not a prototype [-Werror,-Wstrict-prototypes]
extern int rl_message ();
                      ^
                       void
5 errors generated.
gmake[3]: *** [Makefile:2823: virsh-virsh.o] Error 1

Fix that by adding -D_FUNCTION_DEF to READLINE_CFLAGS to fix *Function
related warnings and add a check for stdarg.h so we have HAVE_STDARG_H
defined that's needed by the readline headers to use proper rl_message
declaration.

Bug report on the readline mailing list:

 http://lists.gnu.org/archive/html/bug-readline/2017-05/msg00004.html