Improvements of paths searches in configure and cmake

JIRA: sft.its.cern.ch/jira/browse/ROOT-7468
(ROOT 5.34 and 6.x) Could you, please, add two directories to the list of these automatically searched by the “configure” and “cmake” (i.e. “pythia8incdir” for Pythia8):
/usr/include/Pythia8
/usr/include/pythia8

JIRA: sft.its.cern.ch/jira/browse/ROOT-7480
(ROOT 6.x) Could you, please, add the following directory to the list of these automatically searched by the “configure” and “cmake” (i.e. “occincdir” for GEOCAD / OpenCascade):
/usr/include/oce