boot-time,readutmp: remove -lrt usage This code uses clock-relevant functions only on platforms that do not need -lrt. * m4/readutmp.m4 (gl_READUTMP): Do not require gl_CLOCK_TIME or add CLOCK_TIME_LIB to READUTMP_LIB. * modules/boot-time (Link): No need to link with CLOCK_TIME_LIB.