package/cegui: use plain assignemnt for first _CONF_OPTS

package/cegui: use plain assignemnt for first _CONF_OPTS

Commit 689b9c1a7cf5 (package/cegui: disable xerces support) added
an unconditional assignment to _CONF_OPTS before all the conditional
ones, but used the append-assignment instead of the traditional plain
assignment.

Fix that by removing the append-assignment.

Use that opportunity to also move the first item of this multi-line
assignment, to its own line.

Signed-off-by: Bartosz Bilas <b.bilas@grinn-global.com>
[yann.morin.1998@free.fr:
  - reference the exact commit that introduce the issue
  - also move the first item to its own line
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>