Replaced config setup to fix #2 and use lib meta data instead.

build/root.build

@@ -10,4 +10,10 @@ mxx{*}: extension = cc
 #
 test.target = $cxx.target
 
-include $src_root/build/config-options.build2
+
+##############################
+# Project-specific options:
+
+# Set to true to build and provide the `fmt` C++ module.
+config [bool] config.fmt.enable_modules ?= $cxx.features.modules # $($cxx.features.modules == true)
+

fmt/buildfile

@@ -9,6 +9,12 @@ lib{fmt}: include/hxx{**} hxx{version} $imp_libs $int_libs
 lib{fmt}: src/mxx{fmt} : include = ($config.fmt.enable_modules) # `fmt` C++ module only
 lib{fmt}: src/cxx{** -fmt} : include = (!$config.fmt.enable_modules) # no modules only
 
+# Meta data for users
+lib{fmt}:
+{
+  fmt.is_module = [bool] $config.fmt.enable_modules
+}
+
 # Include the generated version header into the distribution (so that we don't
 # pick up an installed one) and don't remove it when cleaning in src (so that
 # clean results in a state identical to distributed).

tests/basics/buildfile

@@ -2,7 +2,7 @@ import libs = fmt%lib{fmt}
 
 ./ : exe{driver} : {cxx}{driver} hxx{tests.inl} $libs testscript{**}
 
-if($config.fmt.enable_modules)
+if $($libs: fmt.is_module)
 {
     ./ : exe{driver-modules} : {cxx}{driver-modules} hxx{tests.inl} $libs testscript{**}
 }

tests/basics/driver-modules.cxx

@@ -1,8 +1,6 @@
 
 #include <cassert>
-import <chrono>;
-import <vector>;
-
+import std;
 import fmt;
 
 #include "tests.inl"

tests/build/root.build

@@ -6,6 +6,7 @@ hxx{*}: extension = hxx
 ixx{*}: extension = ixx
 txx{*}: extension = txx
 cxx{*}: extension = cxx
+mxx{*}: extension = cc
 
 # Every exe{} in this subproject is by default a test.
 #
@@ -15,9 +16,3 @@ exe{*}: test = true
 #
 test.target = $cxx.target
 
-include $src_root/build/config-options.build2
-
-if($config.fmt.enable_modules)
-{
-    mxx{*}: extension = cc
-}