define nimVersion automatically and avoid needing -d:nimVersion140 (#18726)

* define `nimVersion` and avoid needing -d:nimVersion140

* fix changelog

compiler/options.nim

@@ -165,6 +165,7 @@ const
                  cmdCtags, cmdBuildindex}
 
 type
+  NimVer* = tuple[major: int, minor: int, patch: int]
   TStringSeq* = seq[string]
   TGCMode* = enum             # the selected GC
     gcUnselected = "unselected"
@@ -346,6 +347,7 @@ type
     outDir*: AbsoluteDir
     jsonBuildFile*: AbsoluteFile
     prefixDir*, libpath*, nimcacheDir*: AbsoluteDir
+    nimStdlibVersion*: NimVer
     dllOverrides, moduleOverrides*, cfileSpecificOptions*: StringTableRef
     projectName*: string # holds a name like 'nim'
     projectPath*: AbsoluteDir # holds a path like /home/alice/projects/nim/compiler/
@@ -389,6 +391,16 @@ type
     cppCustomNamespace*: string
     vmProfileData*: ProfileData
 
+proc parseNimVersion*(a: string): NimVer =
+  # could be moved somewhere reusable
+  if a.len > 0:
+    let b = a.split(".")
+    assert b.len == 3, a
+    template fn(i) = result[i] = b[i].parseInt # could be optimized if needed
+    fn(0)
+    fn(1)
+    fn(2)
+
 proc assignIfDefault*[T](result: var T, val: T, def = default(T)) =
   ## if `result` was already assigned to a value (that wasn't `def`), this is a noop.
   if result == def: result = val
@@ -560,6 +572,12 @@ proc newPartialConfigRef*(): ConfigRef =
 proc cppDefine*(c: ConfigRef; define: string) =
   c.cppDefines.incl define
 
+proc getStdlibVersion*(conf: ConfigRef): NimVer =
+  if conf.nimStdlibVersion == (0,0,0):
+    let s = conf.symbols.getOrDefault("nimVersion", "")
+    conf.nimStdlibVersion = s.parseNimVersion
+  result = conf.nimStdlibVersion
+
 proc isDefined*(conf: ConfigRef; symbol: string): bool =
   if conf.symbols.hasKey(symbol):
     result = true