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, 2 insertions, 0 deletions

diff --git a/build2/utility b/build2/utility
index a5c4407..29dca77 100644
--- a/build2/utility
+++ b/build2/utility
@@ -5,6 +5,7 @@
 #ifndef BUILD2_UTILITY
 #define BUILD2_UTILITY
 
+#include <tuple>    // make_tuple()
 #include <memory>   // make_shared()
 #include <string>   // to_string()
 #include <utility>  // move(), forward(), declval(), make_pair()
@@ -25,6 +26,7 @@ namespace build2
   using std::declval;
 
   using std::make_pair;
+  using std::make_tuple;
   using std::make_shared;
   using std::make_move_iterator;
   using std::to_string;