build: use the system's ccache by default

ccache 3.x supports direct mode (http://ccache.samba.org/manual.html#_the_direct_mode)
This is not supported in the prebuilt version.
Direct mode has been shown to decrease build time compared to the ccache in prebuilts

Note: this will require your cache to be recreated

Change-Id: Ib146011a3330637c2d306d329fcb39cea058ff12