autotools: Disable eager MSan in ctime_tests

configure.ac

@@ -131,6 +131,20 @@ AC_DEFUN([SECP_TRY_APPEND_DEFAULT_CFLAGS], [
 ])
 SECP_TRY_APPEND_DEFAULT_CFLAGS(SECP_CFLAGS)
 
+ctime_tests_CFLAGS=
+if test "x$GCC" = "xyes"; then
+  # MSan on Clang >=16 reports unitialized memory in function parameters and return values, even if
+  # the uninitalized variable is never actually "used". This is called "eager" checking, and it's
+  # sounds like good idea for normal use of MSan. However, it yields many false positives in the
+  # ctime_tests because many return values depend on secret (i.e., "uninitialized") values, and
+  # we're only interested in detecting branches (which count as "uses") on secret data.
+  #
+  # We need to wrap the argument in a --(start/end)-no-unused-arguments block to suppress a
+  # -Wunused-command-line-argument warning, which clang emits if MSan is not actually enabled, i.e.,
+  # if -fsanitize=memory does not appear on the command line.
+  SECP_TRY_APPEND_CFLAGS([--start-no-unused-arguments -fno-sanitize-memory-param-retval --end-no-unused-arguments], ctime_tests_CFLAGS)
+fi
+
 ###
 ### Define config arguments
 ###