Refactor how ROOT_INCLUDE_PATH is constructed

cmssw-config modified to add $CMSSW_BASE/src and $CMSSW_RELEASE_BASE/src
as the first two paths in ROOT_INCLUDE_PATH. Handler for these two
paths is set to warning, as directories might not exist during CMSSW
compilation.

root-toolfile.spec now creates `root_cxxdefaults' toolfile, which
defines default compiler search paths in ROOT_INCLUDE_PATH and
also sets ROOT_GCC_TOOLCHAIN. `root_cxxdefaults' must be the last in
dependency chain. Any toolfile setting ROOT_INCLUDE_PATH must depend
on `root_cxxdefaults'. This ensures that compiler paths are at the
end of ROOT_INCLUDE_PATH.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>