package/pure-ftpd: add optional support for elfutils

When elfutils was compiled before, pure-ftpd will use it as optional
dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/sbin/pure-ftpd | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [libelf.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libcrypt.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.1]

The build system offers no option to en-/disable elfutils support,
check m4/getloadavg.m4, line 44.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>