configure.ac: check clock_gettime for glib < 2.17

glib < 2.17 doesn't support clock_gettime directly, it is available
via librt.

Change-Id: Ice853d85ffe859b1d4df48b91b050c24d85c861b

configure.ac

@@ -68,6 +68,10 @@ AC_SUBST(LIBRARY_DLSYM)
 AC_CHECK_LIB(execinfo, backtrace, BACKTRACE_LIB=-lexecinfo, BACKTRACE_LIB=)
 AC_SUBST(BACKTRACE_LIB)
 
+# check for old glibc < 2.17 to get clock_gettime
+AC_SEARCH_LIBS([clock_gettime], [rt posix4], [LIBRARY_RT="$LIBS";LIBS=""])
+AC_SUBST(LIBRARY_RT)
+
 AC_ARG_ENABLE(doxygen,
 	[AS_HELP_STRING(
 		[--disable-doxygen],

src/Makefile.am

@@ -12,7 +12,7 @@ endif
 
 lib_LTLIBRARIES = libosmocore.la
 
-libosmocore_la_LIBADD = $(BACKTRACE_LIB) $(TALLOC_LIBS)
+libosmocore_la_LIBADD = $(BACKTRACE_LIB) $(TALLOC_LIBS) $(LIBRARY_RT)
 libosmocore_la_SOURCES = timer.c timer_gettimeofday.c timer_clockgettime.c \
 			 select.c signal.c msgb.c bits.c \
 			 bitvec.c bitcomp.c counter.c fsm.c \