Custom SQL driver plugin install step error

When building a custom SQL driver plugin from within Qt's source directory at Src/qtbase/src/plugins/sqldrivers/, the following error occurs from qt_plugins.prf:

/Users/user/Qt/5.9.1/clang_64/mkspecs/features/qt_plugin.prf:59: Cannot write file /mkspecs/modules-inst/qt_plugin_qspatialite.pri: Cannot create parent directory

The plugin builds correctly then encounters this on the install step. I get the same error when trying to build the SQLite driver plugin from its .pro file.

Update: I didn't notice this project error on pro file processing earlier:

Cannot read /Users/user/Qt/5.9.1/Src/qtbase/src/plugins/sqldrivers/qtsqldrivers-config.pri: No such file or directory

This issue may be due to an incorrect path for qtsqldrivers-config.pri.

The include was added to the SQLite sqldriver plugin project between Qt 5.7 and Qt 5.8:

include($$OUT_PWD/../qtsqldrivers-config.pri)