Back to requiring "Perl version 5.6.1 (or later)" [PR82856]

With Subversion r265695 (Git commit 22e05272)
"Update GCC to autoconf 2.69, automake 1.15.1 (PR bootstrap/82856)" we're back
to normal; per Automake 1.15.1 'configure.ac' still "[...] perl 5.6 or better
is required [...]".

	PR bootstrap/82856
	gcc/
	* doc/install.texi (Perl): Back to requiring "Perl version 5.6.1 (or
	later)".