Compile flag to not include system headers

Some builds don't require system headers: crypto only and single
threaded.
WOLFSSL_NO_SYSTEM_HEADERS stops wolfSSL including system headers.

wolfssl/wolfcrypt/wc_port.h

@@ -59,7 +59,12 @@
 #endif /* WOLFSSL_LINUXKM */
 
 /* THREADING/MUTEX SECTION */
-#ifdef USE_WINDOWS_API
+#if defined(WOLFSSL_NO_SYSTEM_HEADERS)
+    #ifndef SINGLE_THREADED
+        #error "Configure as single threaded."
+    #endif
+    /* No system headers required for build. */
+#elif defined(USE_WINDOWS_API)
     #if defined(WOLFSSL_PTHREADS)
         #include <pthread.h>
     #endif