BUILDING.md: the PLUGIN_QUALIFY_BUILD tier needs omx-contract headers on a desk without the package #112

The real-binary tier and tests/sabotage.py build_probe (:487) compile without CONTRACT_CFLAGS. That works on CI, where omx-contract is installed system-wide; a desk without it needs C_INCLUDE_PATH. Add a line to BUILDING.md, or use pkg-config in both.

Found in the review of #108.