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/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>