Fixes for building with old cmake

I needed few minor fixes to be able to build LHCb/master with old cmake. That includes:

• Python 2to3 migration issue (see !44 (closed))
• removed CMAKEFLAGS since it was causing configure to fail, and -C...cache_preload.cmake is passed anyway (it's being passed twice in the old system, see https://lhcb-nightlies.web.cern.ch/nightly/lhcb-master/1382/LHCb/x86_64-centos7-gcc9-opt/build#configure_2)
• passing CMAKE_USE_CCACHE from slot configuration to the build functions
• fix patching the log filename (to account for the fact that make commands are run from e.g. /workspace/LHCb/ directory not /workspace/ as in the new cmake)

mentioned in merge request !44 (closed)

assigned to @clemenci

mentioned in commit abd7b8ca

merged