package/vim: add optional support for gpm

When gpm was compiled before, vim will use it as optional dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/bin/vim | grep NEEDED | sort
 0x0000000000000001 (NEEDED)             Shared library: [libacl.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libdl.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libgpm.so.2]
 0x0000000000000001 (NEEDED)             Shared library: [libintl.so.8]
 0x0000000000000001 (NEEDED)             Shared library: [libm.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libncurses.so.5]

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