Mac build broken by opcode_mix_launcher

Yet another failure we did not notice due to the Travis email bug #3031 (closed): https://travis-ci.org/DynamoRIO/dynamorio/jobs/427939922

debug-internal-32: **** 4 build errors ****
	clang: error: linker command failed with exit code 1 (use -v to see invocation)
	make[2]: *** [clients/bin32/opcode_mix_launcher] Error 1
	make[1]: *** [clients/drcachesim/CMakeFiles/opcode_mix_launcher.dir/all] Error 2
	make: *** [all] Error 2
release-external-32: **** 4 build errors ****
	clang: error: linker command failed with exit code 1 (use -v to see invocation)
	make[2]: *** [clients/bin32/opcode_mix_launcher] Error 1
	make[1]: *** [clients/drcachesim/CMakeFiles/opcode_mix_launcher.dir/all] Error 2
	make: *** [all] Error 2