When building the script directory using a CMake-based build, both the
variables in config.h.cmake (including PLUGINDIR) and the variables in
CMakeList.txt (which includes pkgplugindir).

However, for autotools-based builds, only pkgplugindir is substituted,
which means that the plugin-path is not substituted.

This patch solves the problem by using pkgplugindir, which works on both
CMake-based and autotools-based builds, instead of PLUGINDIR.