meson: hide C API if Xorg is disabled (like autotools)

When building only Xwayland using Meson (example) some files are always installed. This causes package conflict if Xwayland is built separately from Xorg. Let's extend what glx/glamor/exa already do by mimicking autotools behavior.

List of affected files

include/xorg/compositeext.h
include/xorg/damage.h
include/xorg/damagestr.h
include/xorg/dbestruct.h
include/xorg/dri3.h
include/xorg/fb.h
include/xorg/fboverlay.h
include/xorg/fbpict.h
include/xorg/fbrop.h
include/xorg/geext.h
include/xorg/geint.h
include/xorg/glyphstr.h
include/xorg/mi.h
include/xorg/micmap.h
include/xorg/micoord.h
include/xorg/migc.h
include/xorg/miline.h
include/xorg/mioverlay.h
include/xorg/mipict.h
include/xorg/mipointer.h
include/xorg/mipointrst.h
include/xorg/mistruct.h
include/xorg/misync.h
include/xorg/misyncfd.h
include/xorg/misyncshm.h
include/xorg/misyncstr.h
include/xorg/mizerarc.h
include/xorg/panoramiX.h
include/xorg/panoramiXsrv.h
include/xorg/picture.h
include/xorg/picturestr.h
include/xorg/present.h
include/xorg/presentext.h
include/xorg/randrstr.h
include/xorg/rrtransform.h
include/xorg/shadow.h
include/xorg/shmint.h
include/xorg/syncsdk.h
include/xorg/vndserver.h
include/xorg/wfbrename.h
include/xorg/xace.h
include/xorg/xacestr.h
include/xorg/xorg-server.h
include/xorg/xvdix.h
include/xorg/xvmcext.h
share/aclocal/xorg-server.m4