build: use CMAKE_C_COMPILER_LAUNCHER for ccache

build: use CMAKE_C_COMPILER_LAUNCHER for ccache

In some situations, CMake will find ccache in /usr/bin but /usr/bin
might not present in PATH. The former fix for this was to place the
ccache configuration logic before the project() declaration, but since
CMake 3.4 there is a new variable to be used which handles this case.

For the original problem, see also
https://crascit.com/2016/04/09/using-ccache-with-cmake/

Type: fix
Signed-off-by: Guillaume Solignac <gsoligna@cisco.com>
Change-Id: Ie026e02b2b06e2dca2d62da5fea7b1a104bcc7c3