Add support for prepend/append in target type/pattern-specific vars

Semantically, these are similar to variable overrides and are essentially
treated as "templates" that are applied on lookup to the "stem" value that is
specific to the target type/name. For example:

x = [string] a
file{f*}: x =+ b

sub/:
{
  file{*}: x += c

  print $(file{foo}:x)  # abc
  print $(file{bar}:x)  # ac
}

1 files changed, 21 insertions, 15 deletions

diff --git a/build2/variable.txx b/build2/variable.txx
index a3511d4..a86a936 100644
--- a/build2/variable.txx
+++ b/build2/variable.txx
@@ -54,13 +54,15 @@ namespace build2
       catch (const invalid_argument&) {} // Fall through.
     }
 
-    diag_record dr (error);
+    {
+      diag_record dr (error);
 
-    dr << "invalid " << value_traits<T>::value_type.name
-       << " value '" << ns << "'";
+      dr << "invalid " << value_traits<T>::value_type.name
+         << " value '" << ns << "'";
 
-    if (var != nullptr)
-      dr << " in variable " << var->name;
+      if (var != nullptr)
+        dr << " in variable " << var->name;
+    }
 
     throw failed ();
   }
@@ -84,13 +86,15 @@ namespace build2
       catch (const invalid_argument&) {} // Fall through.
     }
 
-    diag_record dr (error);
+    {
+      diag_record dr (error);
 
-    dr << "invalid " << value_traits<T>::value_type.name
-       << " value '" << ns << "'";
+      dr << "invalid " << value_traits<T>::value_type.name
+         << " value '" << ns << "'";
 
-    if (var != nullptr)
-      dr << " in variable " << var->name;
+      if (var != nullptr)
+        dr << " in variable " << var->name;
+    }
 
     throw failed ();
   }
@@ -114,13 +118,15 @@ namespace build2
       catch (const invalid_argument&) {} // Fall through.
     }
 
-    diag_record dr (error);
+    {
+      diag_record dr (error);
 
-    dr << "invalid " << value_traits<T>::value_type.name
-       << " value '" << ns << "'";
+      dr << "invalid " << value_traits<T>::value_type.name
+         << " value '" << ns << "'";
 
-    if (var != nullptr)
-      dr << " in variable " << var->name;
+      if (var != nullptr)
+        dr << " in variable " << var->name;
+    }
 
     throw failed ();
   }