Makefile: support both cross-platform and non-cross installers

In non-cross mode, `-C` needs to go after `-G` and `-X` when setting
up a "bundle" directory to turn into an installer, because that mode
needs to use foreign libraries (such as SQLite) at build time, and it
can use the instances that are being set up for the installer.

Meanwhile, improve the advice for setting `PLAIN_RACKET` to use `-C`
for a cross-platform build mode, even though things tend to work
anyway without it.

INSTALL.txt

@@ -196,10 +196,14 @@ libraries. See the documentation for `raco setup' for information on
 the options.
 
 For cross compilation, add configuration options to
-`CONFIGURE_ARGS_qq="..."' as described in the "README" of "racket/src",
-but also add a `PLAIN_RACKET=...' argument for the top-level makefile
-to specify the same executable as in an `--enable-racket=...' for
-`configure'.
+`CONFIGURE_ARGS_qq="..."' as described in the "README" of
+"racket/src", but also add a `PLAIN_RACKET=...' argument for the
+top-level makefile to specify the same executable as in an
+`--enable-racket=...' for `configure'. In general, the `PLAIN_RACKET`
+setting should have the form `PLAIN_RACKET="... -C"` to ensure that
+cross-compilation mode is used and that any foreign libraries needed
+for build time can be found, but many cross-compilation scenarios work
+without `-C`.
 
 Installing Packages
 -------------------

Makefile

@@ -335,7 +335,7 @@ REMOTE_USER_AUTO = --catalog $(SVR_CAT) $(USER_AUTO_OPTIONS)
 REMOTE_INST_AUTO = --catalog $(SVR_CAT) --scope installation $(X_AUTO_OPTIONS)
 CONFIG_MODE_q = "$(CONFIG)" "$(CONFIG_MODE)"
 BUNDLE_CONFIG = bundle/racket/etc/config.rktd
-BUNDLE_RACO_FLAGS = -C -G bundle/racket/etc -X bundle/racket/collects -A bundle/user -l raco
+BUNDLE_RACO_FLAGS = -G bundle/racket/etc -X bundle/racket/collects -C -A bundle/user -l raco
 BUNDLE_RACO = $(PLAIN_RACKET) $(BUNDLE_RACO_FLAGS)
 WIN32_BUNDLE_RACO = $(WIN32_PLAIN_RACKET) $(BUNDLE_RACO_FLAGS)
 IN_BUNDLE_RACO = bundle/racket/bin/raco

pkgs/racket-doc/scribblings/reference/startup.scrbl

@@ -313,7 +313,9 @@ flags:
         @racket[(find-system-path 'config-dir)] and
         @racket[(find-system-path 'collects-dir)] to be the results of
         @racket[(find-system-path 'host-config-dir)] and
-        @racket[(find-system-path 'host-collects-dir)], respectively.}
+        @racket[(find-system-path 'host-collects-dir)], respectively.
+        If @FlagFirst{C} or @DFlagFirst{cross} is provided multiple
+        times, only the first instance has an effect.}
 
   @item{@FlagFirst{N} @nonterm{file} or @DFlagFirst{name}
         @nonterm{file} : sets the name of the executable as reported

racket/src/racket/cmdline.inc

@@ -1311,11 +1311,13 @@ static int run_from_cmd_line(int argc, char *_argv[],
           was_config_flag = 1;
             break;
         case 'C':
-          cross_compile = 1;
-          scheme_set_cross_compile_mode(1);
-          was_config_flag = 1;
-          host_config_path = config_path;
-          host_collects_path = collects_path;
+          if (!cross_compile) {
+            cross_compile = 1;
+            scheme_set_cross_compile_mode(1);
+            was_config_flag = 1;
+            host_config_path = config_path;
+            host_collects_path = collects_path;
+          }
 	  break;
 	case 'U':
 	  scheme_set_ignore_user_paths(1);