gimp: fix gimptool buildpaths errors

gimptool is used in GIMP plugin compilation. It does need a CC
definition, provide it as the current CC with "--sysroot=..." removed.

gimptool also need the gimp .pc file which is in gimp-dev. Split
gimptool in its own package to avoid circular dependencies.

Signed-off-by: Yoann Congal <yoann.congal@smile.fr>
Signed-off-by: Khem Raj <raj.khem@gmail.com>