redoconf: clock_gettime() detection needs to depend on -lrt.

More generally, default.func.rc.od wasn't super useful because you
couldn't specify either a header file or library dependencies.  Drop it
and make an explicit clock_gettime.rc.od.  As a bonus, this also checks
that CLOCK_MONOTONIC exists as expected.

Reported-by: Wayne Scott <wsc9tt@gmail.com>

redoconf/rc/default.func.rc.od

@@ -1,13 +0,0 @@
-. ./redoconf.rc
-rc_include rc/CC.rc
-
-base="${1#*/}"
-f="${base%.func.rc}"
-F=$(echo "$f" | tr 'a-z.' 'A-Z_')
-
-if rc_compile cc link "void $f(); void test() { $f(); }"; then
-	rc_replaceln "HAVE_$F" 1
-else
-	rc_replaceln "HAVE_$F" ""
-fi
-rc_save