Give better error messages for semi-portable parts
Previously it was failing with errors about headers not found, which is
suboptimal in terms of clarity. Now give a clean error with pointer to the
documentation.
Do the checks in the .c files rather than check_config.h as it keeps them
closer to the platform-specific implementations.
diff --git a/library/timing.c b/library/timing.c
index 5d8b25b..a7c7ff0 100644
--- a/library/timing.c
+++ b/library/timing.c
@@ -38,6 +38,11 @@
#if !defined(MBEDTLS_TIMING_ALT)
+#if !defined(unix) && !defined(__unix__) && !defined(__unix) && \
+ !defined(__APPLE__) && !defined(_WIN32)
+#error "This module only works on Unix and Windows, see MBEDTLS_TIMING_C in config.h"
+#endif
+
#ifndef asm
#define asm __asm
#endif
