Remove syscall wrappers around clock_gettime, closes 12091

12091 "Direct use of __NR_clock_gettime is not time64-safe".

function                                             old     new   delta
runsv_main                                          1698    1712     +14
startservice                                         378     383      +5
get_mono                                              31      25      -6
date_main                                            932     926      -6
gettimeofday_ns                                       17       -     -17
------------------------------------------------------------------------------
(add/remove: 0/1 grow/shrink: 2/2 up/down: 19/-29)            Total: -10 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/Makefile.flags b/Makefile.flags
index 6f6142c..bea4647 100644
--- a/Makefile.flags
+++ b/Makefile.flags
@@ -129,10 +129,12 @@
 # fall back to using a temp file:
 CRYPT_AVAILABLE := $(shell echo 'int main(void){return 0;}' >crypttest.c; $(CC) $(CFLAGS) -lcrypt -o /dev/null crypttest.c >/dev/null 2>&1 && echo "y"; rm crypttest.c)
 ifeq ($(CRYPT_AVAILABLE),y)
-LDLIBS += m crypt
+LDLIBS += m rt crypt
 else
-LDLIBS += m
+LDLIBS += m rt
 endif
+# libm may be needed for dc, awk, ntpd
+# librt may be needed for clock_gettime()
 
 # libpam may use libpthread, libdl and/or libaudit.
 # On some platforms that requires an explicit -lpthread, -ldl, -laudit.