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 by up to 40x, compared to preprocessor mode used in ccache 2.4, which is substantially less

Change-Id: I224de8e5bc53bbe18a7a02fc811324307cf3ab8c
Revert "build: use the system's ccache by default"

This reverts commit 48b56099fcb1941757db100a0be8c516f9a03111.

Change-Id: I800e700adbc1d3dc3399999cef8fca6bff87ef75