Many users seem to have difficulty understanding that if they have installed the C driver to a given $prefix that is not among those prefixes that are automatically searched by pkgconfig, that they must set PKG_CONFIG_PATH to $prefix/lib/pkgconfig.

In most cases, users are installing both the C driver and the C++ driver to the same $prefix.

Perhaps we should automatically add CMAKE_INSTALL_PREFIX/lib/pkgconfig to the PKG_CONFIG_PATH in the environment.

In particular, I would do this if:

• There is no PKG_CONFIG_PATH set in the environment when CMake is invoked (i.e. don't mess with it if the user set it)
• The user has customized CMAKE_INSTALL_PREFIX when invoking the C++ driver CMake (so they are asking to install somewhere other than the default build/install).
• The user has not set LIBBSON_DIR and LIBMONGOC_DIR.

That seems pretty safe to do, and might stop a lot of the "my C++ driver build can't find the C driver" sort of tickets or user group posts.