make JS trunc polyfill opt-in, closes nim-lang#16144 (nim-lang#19183)

changelog.md

@@ -3,7 +3,13 @@
 
 ## Changes affecting backward compatibility
 
-
+- The `Math.trunc` polyfill for targeting Internet Explorer was
+  previously emitted for every JavaScript output file except if
+  the symbol `nodejs` was defined via `-d:nodejs`. Now, it is only
+  emitted if the symbol `nimJsMathTruncPolyfill` is defined. If you are
+  targeting Internet Explorer, you may choose to enable this option
+  or define your own `Math.trunc` polyfill using the [`emit` pragma](https://nim-lang.org/docs/manual.html#implementation-specific-pragmas-emit-pragma). Nim uses
+  `Math.trunc` for the division and modulo operators for integers.
 
 ## Standard library additions and changes
 

lib/system/jssys.nim

@@ -763,13 +763,13 @@ proc nimParseBiggestFloat(s: string, number: var BiggestFloat, start: int): int
 
 # Workaround for IE, IE up to version 11 lacks 'Math.trunc'. We produce
 # 'Math.trunc' for Nim's ``div`` and ``mod`` operators:
-const jsMathTrunc = """
+when defined(nimJsMathTruncPolyfill):
+  {.emit: """
 if (!Math.trunc) {
   Math.trunc = function(v) {
     v = +v;
     if (!isFinite(v)) return v;
     return (v - v % 1) || (v < 0 ? -0 : v === 0 ? v : 0);
   };
 }
-"""
-when not defined(nodejs): {.emit: jsMathTrunc .}
+""".}