os-compatibility: define CLOCK_BOOTTIME when not available
CLOCK_BOOTTIME is supposed to be provided by the C library headers, but uClibc 0.9.33.2 does not provide it. Instead of depending on an uClibc patch, let's make Weston define it to the right value when not already defined by the C library.
Signed-off-by: Thomas Petazzoni email@example.com
Signed-off-by: Fabrice Fontaine firstname.lastname@example.org