Implement optional module loading

The syntax is:

using? cli

Now each module use results in two bool variables: <module>.loaded and
<module>.configured.

Also implement variable visibility (the above two variables are limited
to project).

3 files changed, 13 insertions, 9 deletions

diff --git a/build/config/module b/build/config/module
index 58d9814..530fa7b 100644
--- a/build/config/module
+++ b/build/config/module
@@ -6,15 +6,17 @@
 #define BUILD_CONFIG_MODULE
 
 #include <build/types>
+#include <build/utility>
+
 #include <build/module>
 
 namespace build
 {
   namespace config
   {
-    extern "C" void
+    extern "C" bool
     config_init (
-      scope&, scope&, const location&, std::unique_ptr<module>&, bool);
+    scope&, scope&, const location&, unique_ptr<module>&, bool, bool);
   }
 }
 
diff --git a/build/config/module.cxx b/build/config/module.cxx
index bb787a0..b95c6c7 100644
--- a/build/config/module.cxx
+++ b/build/config/module.cxx
@@ -24,12 +24,13 @@ namespace build
     //
     static const path config_file ("build/config.build");
 
-    extern "C" void
+    extern "C" bool
     config_init (scope& r,
                  scope& b,
                  const location& l,
                  std::unique_ptr<module>&,
-                 bool first)
+                 bool first,
+                 bool)
     {
       tracer trace ("config::init");
 
@@ -39,7 +40,7 @@ namespace build
       if (!first)
       {
         warn (l) << "multiple config module initializations";
-        return;
+        return true;
       }
 
       const dir_path& out_root (r.out_path ());
@@ -65,6 +66,8 @@ namespace build
 
       if (file_exists (f))
         source (f, r, r);
+
+      return true;
     }
   }
 }
diff --git a/build/config/utility b/build/config/utility
index 9a5dc5e..ece7a88 100644
--- a/build/config/utility
+++ b/build/config/utility
@@ -42,8 +42,7 @@ namespace build
               const T& default_value,
               bool override = false)
     {
-      return required (
-        root, variable_pool.find (name), default_value, override);
+      return required (root, var_pool.find (name), default_value, override);
     }
 
     inline std::pair<std::reference_wrapper<const value>, bool>
@@ -68,7 +67,7 @@ namespace build
     inline const value&
     optional (scope& root, const std::string& var)
     {
-      return optional (root, variable_pool.find (var));
+      return optional (root, var_pool.find (var));
     }
 
     // As above but assumes the value is dir_path and makes it
@@ -81,7 +80,7 @@ namespace build
     inline const value&
     optional_absolute (scope& root, const std::string& var)
     {
-      return optional_absolute (root, variable_pool.find (var));
+      return optional_absolute (root, var_pool.find (var));
     }
 
     // Check whether there are any variables specified from the