autoconf: portability fixes

The rework of the host tools discovery caused autoconf to embed
absolute paths to staging_dir/host/bin/perl into the shebang
of the generated autoconfig executables.

Switch to the portable "/usr/bin/env perl" and enable global
warnings through "$^W" instead of the "-w" interpreter argument.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

SVN-Revision: 44971