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.

include/mbedtls/config.h

@@ -1897,11 +1897,15 @@
 /**
  * \def MBEDTLS_NET_C
  *
- * Enable the TCP/IP networking routines.
+ * Enable the TCP and UDP over IPv6/IPv4 networking routines.
+ *
+ * \note This module only works on Unix (including Linux, BSD and OS X) and
+ * Windows. For other platforms, you'll want to disable it, and write your
+ * own networking callbacks to be passed to \c mbedtls_ssl_set_bio().
  *
  * Module:  library/net.c
  *
- * This module provides TCP/IP networking routines.
+ * This module provides networking routines.
  */
 #define MBEDTLS_NET_C
 
@@ -2264,7 +2268,14 @@
 /**
  * \def MBEDTLS_TIMING_C
  *
- * Enable the portable timing interface.
+ * Enable the semi-portable timing interface.
+ *
+ * \note The provided implementation only works on Unix (including Linux, BSD
+ * and OS X) and Windows. On other platforms, you can either disable that
+ * module and provide your own implementations of the callbacks needed by
+ * \c mbedtls_ssl_set_timer_cb() for DTLS, or leave it enabled and provide
+ * your own implementation of the whole module by setting
+ * \c MBEDTLS_TIMING_ALT in the current file.
  *
  * Module:  library/timing.c
  * Caller:  library/havege.c

library/entropy_poll.c

@@ -39,6 +39,12 @@
 #endif
 
 #if !defined(MBEDTLS_NO_PLATFORM_ENTROPY)
+
+#if !defined(unix) && !defined(__unix__) && !defined(__unix) && \
+    !defined(__APPLE__) && !defined(_WIN32)
+#error "Platform entropy sources only work on Unix and Windows, see MBEDTLS_NO_PLATFORM_ENTROPY in config.h"
+#endif
+
 #if defined(_WIN32) && !defined(EFIX64) && !defined(EFI32)
 
 #if !defined(_WIN32_WINNT)

library/net.c

@@ -27,6 +27,11 @@
 
 #if defined(MBEDTLS_NET_C)
 
+#if !defined(unix) && !defined(__unix__) && !defined(__unix) && \
+    !defined(__APPLE__) && !defined(_WIN32)
+#error "This module only works on Unix and Windows, see MBEDTLS_NET_C in config.h"
+#endif
+
 #include "mbedtls/net.h"
 
 #include <string.h>

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