gpm: avoid use of host emacs

configure looks for emacs installation to byte compile .el files. This may
break the build if the host installed emacs is missing needed dependencies.
Since we don't (yet?) package emacs for target, just disable detection of
emacs.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>