devel/boost-libs: oops, work around ${CC} usage after r430247

$ make -V '${MAKE_ARGS:M--toolset*}'
/bin/sh: clang39: not found
make: "/usr/ports/Mk/Uses/compiler.mk" line 69: warning: "clang39 --version" returned non-zero status
make: "/usr/ports/Mk/Uses/compiler.mk" line 120: warning: "clang++39 -### /dev/null 2>&1" returned non-zero status
--toolset=gcc

PR: 213867
MFH: 2016Q4 (r430247 followup)