Allow setting variable using += without prior =

[Simon Josefsson]

If I do:

EXTRA_DIST += FAQ

I get:

Makefile.am:24: EXTRA_DIST must be set with `=' before using `+='

This seem unnecessary rigid.

The background:

I'm currently working on "gnulib-tool" for gnulib, which copies
various compatibility files, and creates Makefile.am and configure.ac
snippets, for use in other projects.

I want to create, e.g., a gnulib.mk, which would say:

EXTRA_DIST += xalloc.h

And then have the project's Makefile.am say

include gnulib.mk

The problem is that currently this would require Makefile.am to set
'EXTRA_DIST =' first.  If gnulib.mk were to include the same
statement, Makefile.am couldn't flexibly set EXTRA_DIST as it wish.

Would it be possible to change the error message into assuming the
variable is simply empty?  Please enlighten me if there is some
obvious problem doing that, as I can't see it.

Thanks,
Simon