This is due to the current quoting of those variables when passed to `include_directories()`. These variables may (and soon will, see below) be empty in the case where the include directory is a system include path that is implicitly included.
I ran into this issue when trying to build thrift using gcc-6. gcc-6 doesn't work with includes that are part of the platform/system include path (see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70129 for one mention). To resolve this, `FindBoost.cmake` will be adjusted to avoid adding those types of directories to Boost_INCLUDE_DIRS.