devel/cpp-netlib: unbreak with boost 1.63

CMakeFiles/mime-roundtrip.dir/mime-roundtrip.cpp.o: In function `main':
libs/mime/test/mime-roundtrip.cpp:(.text+0x16d7a): undefined reference to `init_unit_test()'

PR: 215598
Reported by: antoine (via exp-run)
Approved by: portmgr blanket