fix some issues with --backend (#14363)

* fix some issues with --backend
* fix https://github.com/timotheecour/Nim/issues/175; improve upon #14306

compiler/extccomp.nim

@@ -306,13 +306,14 @@ proc isVSCompatible*(conf: ConfigRef): bool =
 proc getConfigVar(conf: ConfigRef; c: TSystemCC, suffix: string): string =
   # use ``cpu.os.cc`` for cross compilation, unless ``--compileOnly`` is given
   # for niminst support
-  let fullSuffix =
-    case conf.backend
-    of backendCpp, backendJs, backendObjc: "." & $conf.backend & suffix
-    of backendC: suffix
-    else:
-      doAssert false
-      ""
+  var fullSuffix = suffix
+  case conf.backend
+  of backendCpp, backendJs, backendObjc: fullSuffix = "." & $conf.backend & suffix
+  of backendC: discard
+  of backendInvalid:
+    # during parsing of cfg files; we don't know the backend yet, no point in
+    # guessing wrong thing
+    return ""
 
   if (conf.target.hostOS != conf.target.targetOS or conf.target.hostCPU != conf.target.targetCPU) and
       optCompileOnly notin conf.globalOptions: