bug#29931: 27.0.50; Slightly suboptimal build behavior on macOS

[Philipp <p.stephani2@gmail.com>]

There are a few small bugs when building on macOS and not passing the
right configure options.

1. When running configure without options, the build fails with an error

xml.c:26:10: fatal error: 'libxml/tree.h' file not found
#include <libxml/tree.h>
         ^~~~~~~~~~~~~~~

Apparently configure detects libxml2 to be present, but doesn't set the
correct include path.

2. When building with --without-libxml2, the build succeeds, but gives
warnings like

  GEN      ../../info/auth.info
./../emacs/docstyle.texi:3: warning: unrecognized encoding name `UTF-8'.

This is because macOS ships an ancient version of makeinfo (4.8).  It's
possible to install a newer version using Homebrew, but that's not in
PATH and therefore not found.  Maybe configure could also search for
makeinfo in the Homebrew directory (/usr/local/opt/texinfo/bin)?


In GNU Emacs 27.0.50 (build 1, x86_64-apple-darwin17.3.0, NS appkit-1561.20 Version 10.13.2 (Build 17C88))
 of 2018-01-01 built on p
Repository revision: d70a841f6a6f8d38620a4025a84c7bd76788531a
Windowing system distributor 'Apple', version 10.3.1561
System Description:  Mac OS X 10.13.2

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
 'configure --without-xml2'

Configured features:
NOTIFY ACL GNUTLS ZLIB TOOLKIT_SCROLL_BARS NS JSON

Important settings:
  value of $LANG: de_DE.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny seq byte-opt gv
bytecomp byte-compile cconv cl-loaddefs cl-lib dired dired-loaddefs
format-spec rfc822 mml easymenu mml-sec password-cache epa derived epg
epg-config gnus-util rmail rmail-loaddefs mm-decode mm-bodies mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047
rfc2045 ietf-drums mm-util mail-prsvr mail-utils elec-pair time-date
tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
mwheel term/ns-win ns-win ucs-normalize mule-util term/common-win
tool-bar dnd fontset image regexp-opt fringe tabulated-list replace
newcomment text-mode elisp-mode lisp-mode prog-mode register page
menu-bar rfn-eshadow isearch timer select scroll-bar mouse jit-lock
font-lock syntax facemenu font-core term/tty-colors frame cl-generic
cham georgian utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao
korean japanese eucjp-ms cp51932 hebrew greek romanian slovak czech
european ethiopic indian cyrillic chinese composite charscript charprop
case-table epa-hook jka-cmpr-hook help simple abbrev obarray minibuffer
cl-preloaded nadvice loaddefs button faces cus-face macroexp files
text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget hashtable-print-readable backquote kqueue cocoa ns
multi-tty make-network-process emacs)

Memory information:
((conses 16 204457 9102)
 (symbols 48 20149 1)
 (miscs 40 43 191)
 (strings 32 28873 1569)
 (string-bytes 1 770488)
 (vectors 16 35127)
 (vector-slots 8 719006 13116)
 (floats 8 48 68)
 (intervals 56 200 0)
 (buffers 992 11))