Let Makefile have contents:

  MY_MAKE_VAR=foo bar

Let makefile-symbol.el have contents:

  (defun myfun ()
    (interactive)
    (message "thing-at-point symbol is %s" (thing-at-point 'symbol)))

Run emacs -Q --load makefile-symbol.el Makefile

M-x myfun

Minibuffer indicates symbol at point is: MY_MAKE_VAR=foo

Expected: MY_MAKE_VAR

In GNU Emacs 24.3.50.6 (x86_64-unknown-linux-gnu, GTK+ Version 2.10.4)
 of 2013-05-23 on redacted
Windowing system distributor `The X.Org Foundation', version 11.0.70101000
System Description:    Red Hat Enterprise Linux Client release 5.4 (Tikanga)

Configured using:
 `configure --prefix=/redacted/user/boreilly/sw/emacs-install-trunk
 --with-gif=no'

Important settings:
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=none
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

Major mode: GNUmakefile

Minor modes in effect:
  tooltip-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

Recent input:
M-x m y f u n <return> M-x r e p o r t <tab> <retu
rn>

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Makefile has auto save data; consider M-x recover-this-file
thing-at-point symbol is MY_MAKE_VAR=foo

Load-path shadows:
None found.

Features:
(shadow sort nadvice gnus-util mail-extr emacsbug message format-spec
rfc822 mml easymenu mml-sec mm-decode mm-bodies mm-encode mail-parse
rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045
ietf-drums mm-util mail-prsvr mail-utils thingatpt make-mode time-date
tooltip ediff-hook vc-hooks lisp-float-type mwheel x-win x-dnd tool-bar
dnd fontset image regexp-opt fringe tabulated-list newcomment lisp-mode
register page menu-bar rfn-eshadow timer select scroll-bar mouse
jit-lock font-lock syntax facemenu font-core frame cham georgian
utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao korean
japanese hebrew greek romanian slovak czech european ethiopic indian
cyrillic chinese case-table epa-hook jka-cmpr-hook help simple abbrev
minibuffer loaddefs button faces cus-face macroexp files text-properties
overlay sha1 md5 base64 format env code-pages mule custom widget
hashtable-print-readable backquote make-network-process dbusbind
dynamic-setting system-font-setting font-render-setting move-toolbar gtk
x-toolkit x multi-tty emacs)