Tune dependencies

Don't depend on srv.c in config.h, but add explicit checks. This is more
in line with other options that only make sense server-side, and also it
allows to test full config minus srv.c more easily.
diff --git a/include/mbedtls/check_config.h b/include/mbedtls/check_config.h
index 2280cab..8dadbe1 100644
--- a/include/mbedtls/check_config.h
+++ b/include/mbedtls/check_config.h
@@ -422,7 +422,7 @@
 #endif
 
 #if defined(MBEDTLS_SSL_DTLS_CLIENT_PORT_REUSE) && \
-    ( !defined(MBEDTLS_SSL_DTLS_HELLO_VERIFY) || !defined(MBEDTLS_SSL_SRV_C) )
+    !defined(MBEDTLS_SSL_DTLS_HELLO_VERIFY)
 #error "MBEDTLS_SSL_DTLS_CLIENT_PORT_REUSE  defined, but not all prerequisites"
 #endif