patch 9.1.0616: filetype: Make syntax highlighting off for MS Makefiles

Problem:  filetype: Make syntax highlighting off for MS Makefiles
Solution: Try to detect MS Makefiles and adjust syntax rules to it.
          (Ken Takata)

Highlighting of variable expansion in Microsoft Makefile can be broken.
E.g.:
2979cfc262/src/Make_mvc.mak (L1331)

Don't use backslash as escape characters if `make_microsoft` is set.
Also fix that `make_no_comments` was not considered if `make_microsoft`
was set.

Also add description for `make_microsoft` and `make_no_comments` to the
documentation and include a very simple filetype test

closes: #15341

Signed-off-by: Christian Brabandt <cb@256bit.org>
Signed-off-by: Ken Takata <kentkt@csc.jp>

runtime/autoload/dist/ft.vim

@@ -532,6 +532,25 @@ export def FTm()
   endif
 enddef
 
+export def FTmake()
+  # Check if it is a Microsoft Makefile
+  unlet! b:make_microsoft
+  var n = 1
+  while n < 1000 && n <= line('$')
+    var line = getline(n)
+    if line =~? '^\s*!\s*\(ifn\=\(def\)\=\|include\|message\|error\)\>'
+      b:make_microsoft = 1
+      break
+    elseif line =~ '^ *ifn\=\(eq\|def\)\>' || line =~ '^ *[-s]\=include\s'
+      break
+    elseif line =~ '^ *\w\+\s*[!?:+]='
+      break
+    endif
+    n += 1
+  endwhile
+  setf make
+enddef
+
 export def FTmms()
   var n = 1
   while n < 20