qdoc: Don't pass -isystem to clang without setting QDOC_PASS_ISYSTEM

This update surrounds the adding of -system parameters to includePaths with
qdoc will not pass any isystem includes to clang.

This update also simplifies some if statements in the help project writer
by using some existing convenience functions in place of comparisons to
enum values.

Change-Id: I35d89501dcb51a98af6a6c3278f0f071ceb78259
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>