Redo extension derivation for file{}, doc{}, and cli{}

We now first check the 'extension' variable, then use the default.

1 files changed, 3 insertions, 2 deletions

diff --git a/build/target b/build/target
index 2aea7ae..69f8f4f 100644
--- a/build/target
+++ b/build/target
@@ -1051,9 +1051,10 @@ namespace build
   const std::string&
   target_extension_fix (const target_key&, scope&);
 
-  // Get the extension from the variable.
+  // Get the extension from the variable or use the default if none set.
+  // Issue diagnostics and fail if the default is NULL.
   //
-  template <const char* var>
+  template <const char* var, const char* def>
   const std::string&
   target_extension_var (const target_key&, scope&);