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/entropy_poll.c b/library/entropy_poll.c
index 25a27be..972ad2a 100644
--- a/library/entropy_poll.c
+++ b/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)
diff --git a/library/net.c b/library/net.c
index a77268c..3b78b6b 100644
--- a/library/net.c
+++ b/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>
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