clean $CPATH before building ROOT to avoid clash between external LLVM loaded as a dependency and internal LLVM used by ROOT

[boegel]

fix for problem reported by @arielzn in https://sft.its.cern.ch/jira/browse/ROOT-10634 when building ROOT 6.20.04 with foss/2019b, where header files of LLVM 9.0.0 that is loaded as a dep for Mesa 19.1.7 (a direct dep for ROOT) are picked up, leading to errors like:

/tmp/root-6.20.04/interpreter/llvm/src/tools/clang/include/clang/Sema/Ownership.h:123:10: error: 'isPodLike' is not a class template
--
struct isPodLike<clang::OpaquePtr<T> > { static const bool value = true; }
;
^~~~~~~~~
/tmp/root-6.20.04/interpreter/llvm/src/tools/clang/include/clang/Sema/Ownership.h:123:40: error: 'llvm::isPodLike' is not a template
struct isPodLike<clang::OpaquePtr<T> >
{ static const bool value = true; };

[boegel]

Also tested with existing easyconfig ROOT-6.14.06-foss-2018b-Python-3.6.6.eb, should be good to go...

[akesandgren]

LGTM

[akesandgren]

Going in, thanks @boegel!