Fixes #351.

src/nimble.nim

@@ -634,7 +634,7 @@ proc execBackend(options: Options) =
   var args = ""
   for path in paths: args.add("--path:\"" & path & "\" ")
   for option in options.action.compileOptions:
-    args.add(option & " ")
+    args.add("\"" & option & "\" ")
 
   let backend =
     if options.action.backend.len > 0:

tests/tester.nim

@@ -453,3 +453,11 @@ suite "can handle two binary versions":
     check exitCode == QuitSuccess
     check output.strip() == "v2"
 
+test "can pass args with spaces to Nim (#351)":
+  cd "binaryPackage/v2":
+    let (output, exitCode) = execCmdEx(nimblePath &
+                                       " c -r" &
+                                       " -d:myVar=\"string with spaces\"" &
+                                       " binaryPackage")
+    checkpoint output
+    check exitCode == QuitSuccess