Make xolox#misc#os#find_vim() configurable

xolox/vim-easytags#54 shows that it can be useful to override the default
logic of xolox#misc#os#find_vim(), even if it is to work around other
problems:

  xolox/vim-easytags#54

README.md

@@ -39,7 +39,7 @@ that I haven't published yet.
 <!-- Start of generated documentation -->
 
 The documentation of the 42 functions below was extracted from
-11 Vim scripts on May 24, 2013 at 00:14.
+12 Vim scripts on June  1, 2013 at 14:57.
 
 ### Handling of special buffers
 
@@ -259,7 +259,9 @@ Returns 1 (true) when on Microsoft Windows, 0 (false) otherwise.
 Returns the program name of Vim as a string. On Windows and UNIX this
 simply returns [v:progname] [progname] while on Mac OS X there is some
 special magic to find MacVim's executable even though it's usually not on
-the executable search path.
+the executable search path. If you want, you can override the value
+returned from this function by setting the global variable
+`g:xolox#misc#os#vim_progname`.
 
 [progname]: http://vimdoc.sourceforge.net/htmldoc/eval.html#v:progname
 

autoload/xolox/misc.vim

@@ -1,7 +1,7 @@
 " The version of my miscellaneous scripts.
 "
 " Author: Peter Odding <peter@peterodding.com>
-" Last Change: May 25, 2013
+" Last Change: June 1, 2013
 " URL: http://peterodding.com/code/vim/misc/
 
-let g:xolox#misc#version = '1.0'
+let g:xolox#misc#version = '1.1'

autoload/xolox/misc/os.vim

@@ -1,7 +1,7 @@
 " Operating system interfaces.
 "
 " Author: Peter Odding <peter@peterodding.com>
-" Last Change: May 25, 2013
+" Last Change: June 1, 2013
 " URL: http://peterodding.com/code/vim/misc/
 
 function! xolox#misc#os#is_win() " {{{1
@@ -13,11 +13,16 @@ function! xolox#misc#os#find_vim() " {{{1
   " Returns the program name of Vim as a string. On Windows and UNIX this
   " simply returns [v:progname] [progname] while on Mac OS X there is some
   " special magic to find MacVim's executable even though it's usually not on
-  " the executable search path.
+  " the executable search path. If you want, you can override the value
+  " returned from this function by setting the global variable
+  " `g:xolox#misc#os#vim_progname`.
   "
   " [progname]: http://vimdoc.sourceforge.net/htmldoc/eval.html#v:progname
   let progname = ''
-  if has('macunix')
+  if exists('g:xolox#misc#os#vim_progname')
+    let progname = g:xolox#misc#os#vim_progname
+  endif
+  if empty(progname) && has('macunix')
     " Special handling for Mac OS X where MacVim is usually not on the $PATH.
     call xolox#misc#msg#debug("vim-misc %s: Trying MacVim workaround to find Vim executable ..", g:xolox#misc#version)
     let segments = xolox#misc#path#split($VIMRUNTIME)
@@ -27,6 +32,7 @@ function! xolox#misc#os#find_vim() " {{{1
     endif
   endif
   if empty(progname)
+    " Default logic.
     call xolox#misc#msg#debug("vim-misc %s: Looking for Vim executable named %s on search path ..", g:xolox#misc#version, string(v:progname))
     let candidates = xolox#misc#path#which(v:progname)
     if !empty(candidates)