package/xterm: Avoid freetype2 path poisoning using imake

package/xterm: Avoid freetype2 path poisoning using imake

When imake is installed on the host, it tries to include
freetype headers from host, so we must override ac_cv_path_IMAKE
to avoid this.

Extract from config.log:

configure:14803: checking if we should use imake to help
configure:14820: result: yes
configure:14829: checking for xmkmf
configure:14846: found /usr/bin/xmkmf
configure:14857: result: /usr/bin/xmkmf
configure:14920: testing Using /usr/bin/xmkmf  ...
configure:15015: testing IMAKE_CFLAGS  -I. -I/usr/include/freetype2

Signed-off-by: Valentin Korenblit <valentin.korenblit@smile.fr>
[Thomas: pass ac_cv_path_IMAKE="" as suggested by Romain Naour.]
Reviewed-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>