print compiler warning about the VM used

why:
  handy to have confirmation about which of the three different VMs
  is activated

nimbus/nimbus.nim

@@ -7,6 +7,9 @@
 # This file may not be copied, modified, or distributed except according to
 # those terms.
 
+import
+  ../nimbus/vm_compile_info
+
 import
   os, strutils, net, options,
   eth/keys, db/[storage_types, db_chain, select_backend],

nimbus/vm_compile_info.nim

@@ -0,0 +1,20 @@
+# Nimbus
+# Copyright (c) 2018 Status Research & Development GmbH
+# Licensed under either of
+#  * Apache License, version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or
+#    http://www.apache.org/licenses/LICENSE-2.0)
+#  * MIT license ([LICENSE-MIT](LICENSE-MIT) or
+#    http://opensource.org/licenses/MIT)
+# at your option. This file may not be copied, modified, or distributed except
+# according to those terms.
+
+when defined(evmc_enabled):
+  {.warning: "*** Compiling with EVMC enabled".}
+
+elif defined(vm2_enabled):
+  {.warning: "*** Compiling with VM2 enabled".}
+
+else:
+  {.warning: "*** Compiling with standard NIM VM enabled".}
+
+{.used.}

tests/all_tests.nim

@@ -5,6 +5,7 @@
 #  * MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
 # at your option. This file may not be copied, modified, or distributed except according to those terms.
 
+import ../nimbus/vm_compile_info
 import macros, strutils, os, unittest2, osproc
 import threadpool