Support cross-compilation with autotools

We use the host system's `fluid` when cross-compiling. This must be
executable as `fluid`, i.e. it must be in the PATH or otherwise
defined, for instance as an alias.

makeinclude.in

@@ -3,17 +3,17 @@
 #
 # Make include file for the Fast Light Tool Kit (FLTK).
 #
-# Copyright 1998-2018 by Bill Spitzak and others.
+# Copyright 1998-2020 by Bill Spitzak and others.
 #
 # This library is free software. Distribution and use rights are outlined in
 # the file "COPYING" which should have been included with this file.  If this
 # file is missing or damaged, see the license at:
 #
-#     http://www.fltk.org/COPYING.php
+#     https://www.fltk.org/COPYING.php
 #
 # Please report all bugs and problems on the following page:
 #
-#      http://www.fltk.org/str.php
+#      https://www.fltk.org/str.php
 #
 
 # FLTK version numbers
@@ -122,6 +122,10 @@ THREADS		= @THREADS@
 # Name of FLUID executable we install
 FLUID		= @FLUID@$(EXEEXT)
 
+# Name and/or path of FLUID executable we use for building:
+# depends on the build type (native or cross-compiler)
+FLUID_BUILD	= @FLUID_BUILD@
+
 # Possible steps for OS X build only
 OSX_ONLY	= @OSX_ONLY@