system.nim cleanup some exported constants which should never have be… (#17909)

* system.nim cleanup some exported constants which should never have been exported

lib/system/ansi_c.nim

@@ -74,7 +74,7 @@ elif defined(haiku):
     SIGPIPE* = cint(7)
     SIG_DFL* = cast[CSighandlerT](0)
 else:
-  when NoFakeVars:
+  when defined(nimscript):
     {.error: "SIGABRT not ported to your platform".}
   else:
     var

lib/system/coro_detection.nim

@@ -0,0 +1,20 @@
+## Coroutine detection logic
+
+template coroutinesSupportedPlatform(): bool =
+  when defined(sparc) or defined(ELATE) or defined(boehmgc) or defined(gogc) or
+    defined(nogc) or defined(gcRegions) or defined(gcMarkAndSweep):
+    false
+  else:
+    true
+
+when defined(nimCoroutines):
+  # Explicit opt-in.
+  when not coroutinesSupportedPlatform():
+    {.error: "Coroutines are not supported on this architecture and/or garbage collector.".}
+  const nimCoroutines* = true
+elif defined(noNimCoroutines):
+  # Explicit opt-out.
+  const nimCoroutines* = false
+else:
+  # Autodetect coroutine support.
+  const nimCoroutines* = false

lib/system/excpt.nim

@@ -146,7 +146,7 @@ proc closureIterSetupExc(e: ref Exception) {.compilerproc, inline.} =
 
 # some platforms have native support for stack traces:
 const
-  nativeStackTraceSupported* = (defined(macosx) or defined(linux)) and
+  nativeStackTraceSupported = (defined(macosx) or defined(linux)) and
                               not NimStackTrace
   hasSomeStackTrace = NimStackTrace or defined(nimStackTraceOverride) or
     (defined(nativeStackTrace) and nativeStackTraceSupported)

lib/system/io.nim

@@ -252,7 +252,7 @@ proc write*(f: File, s: string) {.tags: [WriteIOEffect], benign.} =
       raiseEIO("cannot write string to file")
 {.pop.}
 
-when NoFakeVars:
+when defined(nimscript):
   when defined(windows):
     const
       IOFBF = cint(0)