package/qt5/qt5base: use ccache for building host code

package/qt5/qt5base: use ccache for building host code

qt5 currently doesn't use HOSTCC/HOSTCXX, so it doesn't use ccache
when building all its host code (especially qmake). This means that
even with ccache enabled and a hot cache, it still takes a long time
to build qt5base.

Before this patch, building qt5base takes:

 - 446 seconds with a cold ccache
 - 185 seconds with a hot ccache

This is because the ccache is not used for host code.

After this patch, building qt5base takes:

 - 450 seconds with a cold ccache
 - 15 seconds with a hot ccache

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>