build: Support clock_gettime() provided in librt

In older glibc versions (< 2.17) clock_gettime() is in librt. Add a
check for this to avoid build breakage for programs/libraries that
use libbsd on such systems.

Based-on-patch-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Guillem Jover <guillem@hadrons.org>

configure.ac

@@ -44,6 +44,16 @@ AC_SUBST([TESTU01_LIBS])
 AM_CONDITIONAL([HAVE_LIBTESTU01],
                [test "x$ac_cv_lib_testu01_unif01_CreateExternGenBits" = "xyes"])
 
+# In old glibc versions (< 2.17) clock_gettime() is in librt.
+AS_CASE([$host_os],
+  [*-gnu], [
+    saved_LIBS="$LIBS"
+    AC_SEARCH_LIBS([clock_gettime], [rt], [CLOCK_GETTIME_LIBS="-lrt"])
+    AC_SUBST([CLOCK_GETTIME_LIBS])
+    LIBS="$saved_LIBS"
+  ]
+)
+
 # Checks for header files.
 AC_CHECK_HEADERS([sys/ndir.h sys/dir.h ndir.h dirent.h])
 

src/Makefile.am

@@ -52,6 +52,8 @@ hash/md5hl.c: $(srcdir)/hash/helper.c
 libbsd_la_DEPENDENCIES = \
 	$(libbsd_la_included_sources) \
 	libbsd.map
+libbsd_la_LIBADD = \
+	$(CLOCK_GETTIME_LIBS)
 libbsd_la_LDFLAGS = \
 	-Wl,--version-script=$(srcdir)/libbsd.map \
 	-version-number $(LIBBSD_ABI)