Add the ability to pass a value with the -d flag

This allows the end user to use the {.magic: "IntDefine"/"StrDefine"}
pragmas to pass values into code at compile time.  This has a nice side
effect of also allowing/requiring a default value to be assigned in the
code (see osalloc.nim/StandaloneHeapSize for an example)

compiler/commands.nim

@@ -122,6 +122,13 @@ proc splitSwitch(switch: string, cmd, arg: var string, pass: TCmdLinePass,
   elif switch[i] in {':', '=', '['}: arg = substr(switch, i + 1)
   else: invalidCmdLineOption(pass, switch, info)
 
+proc hasKeyValuePair(arg: string): bool =
+  for i in 0..arg.high:
+    if arg[i] in {':', '='}:
+      return true
+
+  return false
+
 proc processOnOffSwitch(op: TOptions, arg: string, pass: TCmdLinePass,
                         info: TLineInfo) =
   case whichKeyword(arg)
@@ -342,7 +349,11 @@ proc processSwitch(switch, arg: string, pass: TCmdLinePass, info: TLineInfo) =
     discard "allow for backwards compatibility, but don't do anything"
   of "define", "d":
     expectArg(switch, arg, pass, info)
-    defineSymbol(arg)
+    if hasKeyValuePair(arg):
+      splitSwitch(arg, key, val, pass, info)
+      defineSymbol(key, val)
+    else:
+      defineSymbol(arg)
   of "undef", "u":
     expectArg(switch, arg, pass, info)
     undefSymbol(arg)