Properly disable ccache; it's not a CMake variable but a osrm-backend…

… specific one

bootstrap.sh

@@ -87,7 +87,7 @@ function build_osrm() {
       -DCMAKE_LIBRARY_PATH=${MASON_HOME}/lib \
       -DCMAKE_BUILD_TYPE=${TARGET} \
       -DCMAKE_EXE_LINKER_FLAGS="${LINK_FLAGS}" \
-      -DCMAKE_ENABLE_CCACHE=OFF
+      -DENABLE_CCACHE=OFF
     make -j${JOBS} && make install
     popd