Only check for gperf if --enable-keymap

gperf is only used to generate some header file for src/keymap,
and so we should only test for its existence if --enable-keymap
is given.

See:
 https://github.com/gentoo/eudev/issues/50
 https://bugs.gentoo.org/show_bug.cgi?id=452760

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>