wayland-info: disable Linux-only clocks if not available

Previously, building on DragonFly, FreeBSD, NetBSD, OpenBSD failed with:
wayland-info/wayland-info.c:1783:4: error: use of undeclared identifier 'CLOCK_MONOTONIC_RAW'
             [CLOCK_MONOTONIC_RAW] = "CLOCK_MONOTONIC_RAW",
              ^
wayland-info/wayland-info.c:1784:4: error: use of undeclared identifier 'CLOCK_REALTIME_COARSE'
             [CLOCK_REALTIME_COARSE] =       "CLOCK_REALTIME_COARSE",
              ^
wayland-info/wayland-info.c:1785:4: error: use of undeclared identifier 'CLOCK_MONOTONIC_COARSE'
             [CLOCK_MONOTONIC_COARSE] =      "CLOCK_MONOTONIC_COARSE",
              ^
wayland-info/wayland-info.c:1791:40: error: invalid application of 'sizeof' to an incomplete type 'const char *[]'
        if (clk_id < 0 || (unsigned)clk_id >= ARRAY_LENGTH(names))
                                              ^~~~~~~~~~~~~~~~~~~
wayland-info/wayland-info.c:49:33: note: expanded from macro 'ARRAY_LENGTH'
 #define ARRAY_LENGTH(a) (sizeof (a) / sizeof (a)[0])
                                 ^~~

Signed-off-by: Jan Beich <jbeich@FreeBSD.org>