perl: Don't hardcode interpreter path

This is particularly useful on operating systems that don't ship
Perl as part of the base system (eg. FreeBSD) while still working
just as well as it did before on Linux.

In one case (src/rpc/genprotocol.pl) the interpreter path was
missing altogether.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>

build-aux/mock-noinline.pl

@@ -1,4 +1,4 @@
-#!/usr/bin/perl
+#!/usr/bin/env perl
 
 my %noninlined;
 my %mocked;