[Qgis-user] Problems building 2.18.x on Slackware-14.2

[Rich Shepard]

   Using the SlackBuilds.org script for both 2.18.11 and 2.18.12 produces an
error on my system:

-- Found QScintilla2 PyQt module: 2.9.1
CMake Error at /usr/lib/cmake/qjson/QJSONTargets.cmake:16 (ADD_LIBRARY):
   add_library cannot create imported target "qjson" because another target
   with the same name already exists.
Call Stack (most recent call first):
   /usr/lib/cmake/qjson/QJSONConfig.cmake:6 (include)
   src/providers/arcgisrest/CMakeLists.txt:1 (FIND_PACKAGE)

   I've been told this happens when Qt4/PyQt4 and Qt5/PyQt5 are installed on
the same host; removing both version 5 packages removes the error. I need
both versions because different applications require one version or the
other.

   My web search for this error produced hits with other applications on
different distributions.

   Why is having two versions of Qt and PyQt affect compiling qgis and how
can I resolve this problem?

Rich