Have MBEDTLS_TIMING_C require MBEDTLS_HAVE_TIME

Nowadays, the timing module just builds on a function that provides a timer
with millisecond resolution. In terms of platform requirements, this is
almost exactly equivalent to `mbedtls_ms_time()`
provides (`mbedtls_ms_time()` is arguably a little stronger because it is
supposed to last longer than a single timer object, but an application could
start a timer when it starts, so there's no real difference.) So it's a bit
silly that `timing.c` essentially reimplements this. Rely on
`mbedtls_ms_time()` instead.

This is an API break because in Mbed TLS 4.0, it was possible to enable
`MBEDTLS_TIMING_C` without `MBEDTLS_HAVE_TIME`. However, `timing.c` only
provided an implementation for Windows and Unix-like platforms, and on those
platforms, it is very likely that the default implementation of
`MBEDTLS_HAVE_TIME` would also work. (The main exception would be a platform
that has the traditional Unix function `gettimeofday()`, but not the 1990s
novelty `clock_gettime()`.) So make this an official requirement, as a
belated change that really should have gone into 4.0 if we'd taken the time
to dig into it.

Signed-off-by: Gilles Peskine <[email protected]>

library/mbedtls_check_config.h

@@ -363,5 +363,10 @@
 #error  "MBEDTLS_PKCS7_C is defined, but not all prerequisites"
 #endif
 
+#if defined(MBEDTLS_TIMING_C) && \
+    !(defined(MBEDTLS_HAVE_TIME) || defined(MBEDTLS_TIMING_ALT))
+#error "MBEDTLS_TIMING_C requires either MBEDTLS_HAVE_TIME or MBEDTLS_TIMING_ALT"
+#endif
+
 /* *INDENT-ON* */
 #endif /* MBEDTLS_CHECK_CONFIG_H */