WRaster: Fix incorrect use of macro USE_XSHM in installed header
The header "wraster.h" needs different behaviour depending on whether the
support for X Shared Memory extension was enabled or not; but the related
macro USE_XSHM is defined by WindowMaker's configure. After this header
have been installed, the macro is no more useable.
This patch makes the "wraster.h" a generated file, so it will be different
depending on USE_XSHM, but will not make use of the macro itself.
Signed-off-by: Christophe CURIS <christophe.curis@free.fr>