fix cross compilation to a Windows target

Cross compilation of ECL itself:

- The windres program comes with the usual cross compilation target
  prefixes, use AC_CHECK_TOOL to select the right one.
- For c::update-compiler-features, we need to check the features of the
  host system instead of the target because that determines the
  executable prefix of the ecl_min stub.

Cross compilation of user code: We just have to replace some read time
conditionals by runtime checks.

Further notes on Windows cross compilation using mingw:

- Our old copy of libgmp doesn't work, one needs to update that before
  compiling.
- ASDF fails to compile unless XDG_CACHE_HOME is set (it tries and
  fails to find a Windows cache directory if XDG_CACHE_HOME is empty).
- The `windres` program may be located in a separate package from the
  mingw compiler. On debian, I had to install binutils-mingw-w64 and
  gcc-mingw-w64.

src/cmp/cmpbackend-cxx/cmpbackend-cxx.lsp

@@ -375,8 +375,8 @@ filesystem or in the database of ASDF modules."
                     (list (concatenate 'string "/LIBPATH:"
                                        (ecl-library-directory))
                           (concatenate 'string "/IMPLIB:" implib)))))
-    #+mingw32
-    (setf ld-flags (list* "-shared" ld-flags))
+    (when (member :mingw32 *features*)
+      (setf ld-flags (list* "-shared" ld-flags)))
     (linker-cc o-pathname object-files :type :dll
                :ld-flags ld-flags :ld-libs ld-libs)))
 
@@ -399,8 +399,8 @@ filesystem or in the database of ASDF modules."
                      (concatenate 'string "/LIBPATH:"
                                   (ecl-library-directory))
                      (concatenate 'string "/IMPLIB:" implib)))))
-    #+mingw32
-    (setf ld-flags (list* "-shared" "-Wl,--export-all-symbols" ld-flags))
+    (when (member :mingw32 *features*)
+      (setf ld-flags (list* "-shared" "-Wl,--export-all-symbols" ld-flags)))
     (linker-cc o-pathname object-files :type :fasl
                                        :ld-flags ld-flags :ld-libs ld-libs)))
 
@@ -508,7 +508,6 @@ extern int
 }
 ")
 
-#+:win32
 (defconstant +lisp-program-winmain+ "
 #include <windows.h>
 int
@@ -570,7 +569,7 @@ WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdS
                   (main-name nil)
                   (prologue-code "")
                   (epilogue-code (when (eq target :program) '(SI::TOP-LEVEL T)))
-                  #+:win32 (system :console)
+                  (system :console)
                   &aux
                     (*suppress-compiler-messages* (or *suppress-compiler-messages*
                                                       (not *compile-verbose*)))
@@ -678,10 +677,9 @@ output = si_safe_eval(2, ecl_read_from_cstring(lisp_code), ECL_NIL);
          ;; we don't need wrapper in the program, we have main for that
                                         ;(format c-file +lisp-init-wrapper+ wrap-name init-name)
          (format c-file
-                 #+:win32 (ecase system
-                            (:console +lisp-program-main+)
-                            (:windows +lisp-program-winmain+))
-                 #-:win32 +lisp-program-main+
+                 (ecase system
+                   (:console +lisp-program-main+)
+                   (:windows +lisp-program-winmain+))
                  prologue-code init-name epilogue-code)
          (close c-file)
          (compiler-cc c-name o-name)

src/cmp/cmpbackend-cxx/cmppass2-data.lsp

@@ -59,19 +59,18 @@
 (defun data-c-dump (filename)
   (labels ((produce-strings ()
              ;; Only Windows has a size limit in the strings it creates.
-             #-windows
-             (let ((s (data-dump-array)))
-               (when (plusp (length s))
-                 (list s)))
-             #+windows
-             (loop with string = (data-dump-array)
-                   with max-string-size = 65530
-                   with l = (length string)
-                   for i from 0 below l by max-string-size
-                   for this-l = (min (- l i) max-string-size)
-                   collect (make-array this-l :displaced-to string
-                                              :element-type (array-element-type string)
-                                              :displaced-index-offset i)))
+             (if (member :windows *features*)
+                 (loop with string = (data-dump-array)
+                       with max-string-size = 65530
+                       with l = (length string)
+                       for i from 0 below l by max-string-size
+                       for this-l = (min (- l i) max-string-size)
+                       collect (make-array this-l :displaced-to string
+                                           :element-type (array-element-type string)
+                                           :displaced-index-offset i))
+                 (let ((s (data-dump-array)))
+                   (when (plusp (length s))
+                     (list s)))))
            (output-one-c-string (name string stream)
              (let* ((*wt-string-size* 0)
                     (*wt-data-column* 80)