wpkg --install-prefix ...
The --install-prefix option is used along the --build command when creating a set of binary packages from a source package.
This is done at the time we compile the system since it is the time when such paths are saved in the code (some projects do that!)
The prefix ends up in the Install-Prefix field of the control file of the binary packages. It can later be used by the different pkg-config commands supported by wpkg.
By default no installation prefix is added which means everything gets installed under the root directory (/). It is customary, for most projects, to make use of /usr under a Unix system. When creating your own environment, it is usual to make use of the /opt/<company-or-produc-name>/... directory, wpkg does not enforce the path, it just cannot include a parent directory (..).
Note that this option parameter is used as the Build-Number field value when generating binary packages from a source package.