Linux: link against libdl

Linux: link against libdl

Older versions of Rust on Linux, such as that used in Debian 11 in our
CI, require linking against libdl.  Were we linking with Cargo, this
would be included automatically, but since we're not, explicitly set it
in the system-specific config.

This library is part of libc, so linking against it if it happens to be
unnecessary will add no dependencies to the resulting binary.  In
addition, it is provided by both glibc and musl, so it should be
portable to almost all Linux systems.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>