Fixed build by adding librt to build system.

configure.ac

@@ -388,6 +388,17 @@ AC_C_BIGENDIAN
 # Checks for library functions.
 AC_FUNC_FORK
 AC_CHECK_FUNCS([gettimeofday memset socket strchr malloc])
+if test "x$WIN32" != "xyes"; then
+    AC_CHECK_LIB(rt, clock_gettime,
+        [
+            RT_LIBS="-lrt"
+            AC_SUBST(RT_LIBS)
+        ],
+        [
+            AC_MSG_ERROR([required library rt was not found on your system])
+        ]
+    )
+fi
 
 if test "x$BUILD_AV" = "xyes"; then
     AX_PTHREAD(

toxcore/Makefile.inc

@@ -50,6 +50,7 @@ libtoxcore_la_LDFLAGS = $(TOXCORE_LT_LDFLAGS) \
                         $(EXTRA_LT_LDFLAGS) \
                         $(LIBSODIUM_LDFLAGS) \
                         $(NACL_LDFLAGS) \
+                        $(RT_LIBS) \
                         $(WINSOCK2_LIBS)
 
 libtoxcore_la_LIBADD =  $(LIBSODIUM_LIBS) \