Fix link error on Mojave

1f58b4d8 introduced a link error on my
Mojave machine, because making the Boost include path the highest-priority
one was forcing use of the system's libfdt.h.  But the system's libfdt.h
unfortunately lacks the 'extern "C"' linkage modifier, causing link errors
when invoking FDT routines from C++ code.

Fix by making the Boost include path the lowest-priority one.