Setting ACLOCAL_AMFLAGS with ':=' vs '='

[R. Diez]

Hallo there:

If I add this line to my Makefile.am (and I make sure that the 'm4' subdir is 
created beforehand), then it works as intended:

  ACLOCAL_AMFLAGS = -I m4

However, if I use this syntax:

  ACLOCAL_AMFLAGS := -I m4

Then I get the following warning:

  libtoolize: Consider adding `-I m4' to ACLOCAL_AMFLAGS in Makefile.am.

I don't think using ':=' instead of '=' should matter here. Or am I missing 
something? I took a quick look in the Automake manual and did not find anything 
in this respect.

I am using Automake vesion 1.14.1 on Ubuntu 14.04. Some background information 
to this matter:

There are 2 variable flavors in GNU Make: "recursively expanded variables" 
(A=B) and "Simply expanded variables" (A:=B).

I prefer the latter. From the GNU Make manual:

"Simply expanded variables generally make complicated makefile programming more 
predictable because they work like variables in most programming languages. 
They allow you to redefine a variable using its own value (or its value 
processed in some way by one of the expansion functions) and to use the 
expansion functions much more efficiently (see Functions for Transforming 
Text)."

That flavor is now a POSIX standard (with syntax "::="), so it should be 
portable too (at least in the future).

Thanks in advance,
  rdiez