Allow simpler configuration of Poly/ML linker flags

Previously, one had to copy out all of the generated flags and put
them and the addition into one's poly-includes.ML file. Now one can
just provide a definition of EXTRA_POLY_LDFLAGS, e.g.:

  val EXTRA_POLY_LDFLAGS = ["-ld_classic"]

This example is needed with current Poly/ML on current versions of the
Mac C infrastructure (Xcode version 15).

See

  polyml/polyml#194

for some discussion of the issue with Poly/ML.

tools-poly/configure.sml

@@ -237,17 +237,17 @@ in
       ("val POLY_LDFLAGS = [" ^
        String.concatWith ", "
                          (map quote
-                              (if null POLY_LDFLAGS then machine_flags
-                               else POLY_LDFLAGS)) ^
+                              ((if null POLY_LDFLAGS then machine_flags
+                                else POLY_LDFLAGS) @ EXTRA_POLY_LDFLAGS)) ^
        "]\n"),
    "val POLY_LDFLAGS_STATIC =" -->
       ("val POLY_LDFLAGS_STATIC = [" ^
        String.concatWith ", "
                          (map quote
-                              (if null POLY_LDFLAGS_STATIC then
-                                 ("-static" :: machine_flags)
-                               else
-                                 POLY_LDFLAGS_STATIC)) ^
+                              ((if null POLY_LDFLAGS_STATIC then
+                                  ("-static" :: machine_flags)
+                                else
+                                  POLY_LDFLAGS_STATIC) @ EXTRA_POLY_LDFLAGS)) ^
        "]\n"),
    "val CC =" --> ("val CC = "^quote CC^"\n"),
    "val OS ="       --> ("val OS = "^quote OS^"\n"),

tools-poly/smart-configure.sml

@@ -73,6 +73,7 @@ val DOT_PATH = SOME "";
 val MLTON = SOME "";
 val GNUMAKE = "";
 val POLY_LDFLAGS = [] : string list;
+val EXTRA_POLY_LDFLAGS = [] : string list;
 val POLY_LDFLAGS_STATIC = [] : string list;
 
 val _ = let