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>