Clean up curl define

It doesn't have to be win32-specific, we could (for example) use this on
mac as well to build without curl/SN support.

llarp/CMakeLists.txt

@@ -43,6 +43,7 @@ target_link_libraries(lokinet-util PUBLIC
 )
 if(TARGET curl)
   target_link_libraries(lokinet-util PUBLIC curl)
+  target_compile_definitions(lokinet-util HAVE_CURL)
   if(TARGET libcurl_external)
     add_dependencies(lokinet-util libcurl_external)
   endif()

llarp/config/key_manager.cpp

@@ -6,7 +6,7 @@
 #include "crypto/crypto.hpp"
 #include "crypto/types.hpp"
 
-#ifndef _WIN32
+#ifdef HAVE_CURL
 #include <curl/curl.h>
 #endif
 
@@ -230,8 +230,8 @@ namespace llarp
   bool
   KeyManager::loadIdentityFromLokid()
   {
-#if defined(_WIN32) || defined(_WIN64)
-    LogError("service node mode not supported on windows");
+#ifndef HAVE_CURL
+    LogError("this lokinet was not built with service node mode support");
     return false;
 #else
     CURL* curl = curl_easy_init();