turning of the user package registry in cmake

[Niclas Rosenvik <nros%netbsd.org@localhost>]

When packaging packages that uses cmake I have come across packages
that populate the user registry in the home directory giving a warning
at the end that the fake home directory is populated with files. 
Packages that do this are double-conversion, rapidjson and rtmidi in
wip. But there might be more that I don't know about. It can be turned
of by setting CMAKE_EXPORT_NO_PACKAGE_REGISTRY to on or true. Would it
be okay to set this in mk/configure/cmake.mk so that it is turned for
all packages?

Niclas Rosenvik