C_INCLUDE_PATH vs CPLUS_INCLUDE_PATH

I should have been smarter about this, so I'm writing this TIL to hopefully make myself remember that:

1. When you are working with C++ (cxx) libraries, you need to set the $CPLUS_INCLUDE_PATH.

2. That $C_INCLUDE_PATH is different to $CPLUS_INCLUDE_PATH. The first one is for c and the second one is for c++.