build: Remove configure args for Nemo, was used for media art

The media art sources are now in libmediaart, so this switch is redundant now.
build: Remove configure args for GdkPixbuf/Qt* used for mediaart

All of this has now been pushed out to libmediaart and no longer needed
build: Change configure's --disable-libxml2 to --enable-libxml2, like others.

Nothing changes as a result of this, the default is still to enable by
default.
build: Change configure's --disable-unzip-ps-gz-files to --enable* like others.

Nothing changes as a result of this, the default is still to enable by
default
build: Improve description of build switches