More sensible defaults and allow for overriding. Also make PDB installation
optional on Windows if RelWithDebInfo isn't the build type being used.
CMAKE_INSTALL_PREFIX defaults to GStreamer install location on Windows and
/usr/lib on other platforms.
PLUGIN_INSTALL_DIR defaults to CMAKE_INSTALL_PREFIX/gstreamer-1.0
LIBRARY_INSTALL_DIR defaults to CMAKE_INSTALL_PREFIX/
PDB_INSTALL_DIR defaults to CMAKE_INSTALL_PREFIX/
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