automake error: Unescaped left brace in regex is deprecated

Solution 1:

A deprecation warning isn't in-and-of-itself a problem.

The Perl 5.21.1 release notes indicate:

If you want a literal left curly bracket (also called a left brace) in a regular expression pattern, you should now escape it by either preceding it with a backslash ("\{") or enclosing it within square brackets "[{]", or by using \Q; otherwise a deprecation warning will be raised. This was first announced as forthcoming in the v5.16 release; it will allow future extensions to the language to happen.

You could report this bug to the Automake authors, although they may want to retain compatibility with older versions of Perl.