E3-944: Warn if building from install path

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.