docs: fix MAKEFLAGS testing example

[Ralf Wildenhues]

I think it's been reported here before, but I can't find the reference
now.  Anyway, the ranlib example in the manual is wrong because it is
way too simplistic; try, e.g.,
  make --no-print-directory FOO=bart

Below is a patch that might make the section less fitting for what it
was intended, but at least the code has less problems, or so I hope;
also, I think it is useful to have this code snippet in the manual
so people can copy it.

And yes, even the line wrapping cause headaches to get right.

Thanks,
Ralf

2011-01-26  Ralf Wildenhues  <address@hidden>

        * doc/make.texi (Testing Flags): Fix MAKEFLAGS testing example
        and adjusted text to cope with passed variable assignments and
        long options.

Index: doc/make.texi
===================================================================
RCS file: /cvsroot/make/make/doc/make.texi,v
retrieving revision 1.69
diff -u -r1.69 make.texi
--- doc/make.texi       30 Nov 2010 14:48:53 -0000      1.69
+++ doc/make.texi       26 Jan 2011 20:08:49 -0000
@@ -6449,23 +6449,31 @@
 @section Conditionals that Test Flags
 
 You can write a conditional that tests @code{make} command flags such as
address@hidden by using the variable @code{MAKEFLAGS} together with the
address@hidden function
-(@pxref{Text Functions, , Functions for String Substitution and Analysis}).
address@hidden by using the variable @code{MAKEFLAGS} together with the a
+couple of @code{make} functions
+(@pxref{Functions, , Functions for Transforming Text}).
 This is useful when @code{touch} is not enough to make a file appear up
 to date.
 
-The @code{findstring} function determines whether one string appears as a
-substring of another.  If you want to test for the @samp{-t} flag,
-use @samp{t} as the first string and the value of @code{MAKEFLAGS} as
-the other.
+The @code{foreach} function expands text repeatedly for each element of
+a list.  The @code{filter-out} function removes words from text matching
+a pattern.  The @code{findstring} function determines whether one string
+appears as a substring of another.  Finally, the @code{if} function
+expands code conditionally, based on whether the condition is nonempty
+or not.
+
+Now, if you want to test for the @samp{-t} flag, you can use @samp{t} as
+the string to find in @code{MAKEFLAGS}, after filtering out long options
+and variable assignments that might match wrongly
+(@pxref{Options/Recursion}).
 
 For example, here is how to arrange to use @samp{ranlib -t} to finish
 marking an archive file up to date:
 
 @example
 archive.a: @dots{}
-ifneq (,$(findstring t,$(MAKEFLAGS)))
+ifneq (,$(foreach flag,$(filter-out --%,$(MAKEFLAGS)),$(if \
+          $(findstring =,$(flag)),,$(findstring t,$(flag)))))
         +touch archive.a
         +ranlib -t archive.a
 else