build-sys: fix x11 client-only builds, selectively build man/start-pulseaudio-x11

X11 support for client-only builds was broken by 80c0a497.

The current meson config detects and links to X11 libraries if enabled, but only defines HAVE_X11 if the daemon is being built. This means client-only builds with X11 enabled will still link libpulse to X11 libraries, but none of the X11 using code in libpulsecommon is built.

This MR fixes that bug by setting HAVE_X11 when X11 is enabled and the xcb dependency is found, regardless of if the client and/or daemon are being built. For the client this is sufficient as only the xcb dependency is needed. If the daemon is being built additional x11 dependencies required by x11 modules are then also checked for as required.

Additional fix #444 and selectively install the manual page for start-pulseaudio-x11 only if X11 support is enabled.