configure: check for <readline.h> vs <readline/readline.h>

configure: check for <readline.h> vs <readline/readline.h>

The readline docs (man and info pages) indicate that we should use

  #include <readline/readline.h>
  #include <readline/history.h>

Unfortunately, it appears that on a number of plaforms pkg-config
--cflags actually returns a -I value that requires

  #include <readline.h>
  #include <history.h>

So make an even bigger mess in config/configure to accomodate either
possibility.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>