If a user runs the build from the E3_SITEMODS_PATH, then it can happen that
false dependencies get registered due to how the generated .d files are
parsed to determine module dependencies. Specifically, if you have header
file being used that is located at

${E3_SITEMODS_PATH}/e3-module/module/src/header.h

and it is _not_ being installed, then the dependency parsing script will
see a dependency as an absolute path, and so register that your module
depends on the module `e3-module` version `module`, which is obviously
not what was intended.

While we don't want to stricly _prevent_ a user from building there, it
seems wise to _warn_ a user that there may be unintended consequences.