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/
Previous way of creating caps left invalid colorimetry field 0:0:0:0,
which combined with a bug present in GStreamer 1.8 led to GRAY16_LE data
being corrupted when converted to GRAY16_BE (or vice versa). Current
method will properly initialize colorimetry field, avoiding this problem
in 1.8.
