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/ast.nim

@@ -610,7 +610,7 @@ type
     mEqIdent, mEqNimrodNode, mSameNodeType, mGetImpl,
     mNHint, mNWarning, mNError,
     mInstantiationInfo, mGetTypeInfo, mNGenSym,
-    mNimvm
+    mNimvm, mIntDefine, mStrDefine
 
 # things that we can evaluate safely at compile time, even if not asked for it:
 const