compositor: LOG_TIMESTAMP API, set points of interest

Expose a configure time option (--enable-weston-timing). If enabled, this will
expose a macro (LOG_TIMESTAMP) that takes a single string argument, referred to
as the "flag". When WESTON_TIMING is enabled, this macro will emit a timestamp
in a standard format, appending the "flag". This is useful for exploring
wall-clock time stamps at various points of interest during program execution.

By default, the LOG_TIMESTAMP macro is ignored by the preprocessor.

Signed-off-by: Joe Konno <joe.konno@intel.com>