root,roofit: do not use full path to the compiler (g++)

ROOT-5905

If a full path to the compiler (g++) is used, it's hardcoded as
LLVM_CXX define. CheckABICompatibility() does not relocate the
path, thus it fails to find g++ to verify ABI.

If we use only 'g++' then compiler is located from $PATH.

Resolves errors while building dictionaries.

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