Add experimental option.

This defaults to 'yes' for odd-numbered (unstable) versions, 'no' for
even-numbered (unstable) versions.  It controls an EXPERIMENTAL
preprocessor symbol, defined only when experimental is 'yes', and an
EXPERIMENTAL automake conditional.

This is meant as a guard option for experimental, developer-only code
that should be disabled in stable versions.  The immediate application
is fendrin's new pathfinding/tunnel code, which we want to carry in
the main codebase for the remainder of 1.7 but be automatically
disabled in 1.8 betas and 1.8 itself.

Implemented for scons and autotools but not yet cmake.

SConstruct

@@ -22,10 +22,26 @@ if os.path.exists('.scons-option-cache'):
     print "Saved options:", optfile.read().replace("\n", ", ")[:-2]
     optfile.close()
 
+#
+# Get the Wesnoth version number
+#
+
+config_h_re = re.compile(r"^.*#define\s*(\S*)\s*\"(\S*)\".*$", re.MULTILINE)
+build_config = dict( config_h_re.findall(File("src/wesconfig.h").get_contents()) )
+try:
+    version = build_config["VERSION"]
+    print "Building Wesnoth version %s" % version
+except KeyError:
+    print "Couldn't determin the Wesnoth version number, bailing out!"
+    sys.exit(1)
+
 #
 # Build-control options
 #
 
+# Experimental code is enabled by default in unstable (odd minor version).
+experimental_default = (int(version.split(".")[1]) % 2) == 1
+
 opts = Variables('.scons-option-cache')
 
 def OptionalPath(key, val, env):
@@ -59,6 +75,7 @@ opts.AddVariables(
     PathVariable('python_site_packages_dir', 'sets the directory where python modules are installed', "lib/python/site-packages/wesnoth", PathVariable.PathAccept),
     BoolVariable('editor', 'Enable editor', True),
     BoolVariable('lowmem', 'Set to reduce memory usage by removing extra functionality', False),
+    BoolVariable('experimental', 'Enable experimental code for developees only', experimental_default),
     BoolVariable('lua', 'Enable Lua support', True),
     BoolVariable('notifications', 'Enable support for desktop notifications', True),
     BoolVariable('nls','enable compile/install of gettext message catalogs',True),
@@ -380,6 +397,9 @@ for env in [test_env, client_env, env]:
     if env['lowmem']:
         env.Append(CPPDEFINES = "LOW_MEM")
 
+    if env['experimental']:
+        env.Append(CPPDEFINES = "EXPERIMENTAL")
+
     if env['internal_data']:
         env.Append(CPPDEFINES = "USE_INTERNAL_DATA")
 
@@ -573,8 +593,6 @@ env.Precious(uninstall)
 #
 # Making a distribution tarball.
 #
-config_h_re = re.compile(r"^.*#define\s*(\S*)\s*\"(\S*)\".*$", re.MULTILINE)
-build_config = dict( config_h_re.findall(File("src/wesconfig.h").get_contents()) )
 env["version"] = build_config.get("VERSION")
 if 'dist' in COMMAND_LINE_TARGETS:    # Speedup, the manifest is expensive
     def dist_manifest():

configure.ac

@@ -6,7 +6,12 @@
 
 AC_PREREQ([2.60])
 
+#######################################################################
+# Don't forget to change the default value of 'experimental' 
+# to match whether this is a stable or unstable release.
+#######################################################################
 define([WESNOTH_VERSION],[1.7.12+svn])
+experimental_default=yes
 define([WESNOTH_BUGS],[http://bugs.wesnoth.org])
 
 AC_INIT([Battle for Wesnoth], WESNOTH_VERSION, WESNOTH_BUGS, [wesnoth])
@@ -168,6 +173,18 @@ then
 	CPPFLAGS="$CPPFLAGS -DDEBUG_WINDOW_LAYOUT_GRAPHS"
 fi
 
+AC_ARG_ENABLE([experimental],
+              AS_HELP_STRING([--enable-experimental], [enable experimental developer-only code]),
+	      [experimental=$enableval],
+	      [experimental=$experimental_default])
+
+if test "x$experimental" = "xyes"
+then
+    CPPFLAGS="$CPPFLAGS -DUSE_TINY_GUI"
+fi
+
+AM_CONDITIONAL([EXPERIMENTAL], [test "x$experimental" = "xyes"])
+
 DATADIR=$PACKAGE
 AC_ARG_WITH([datadir-name],
             AS_HELP_STRING([--with-datadir-name@<:@=DIR@:>@], [change name of data directory @<:@wesnoth@:>@]),