Implement support for excluded and ad hoc prerequisites

The inclusion/exclusion is controlled via the 'include' prerequisite-specific
variable. Valid values are:

false  - exclude
true   - include
adhoc  - include but treat as an ad hoc input

For example:

lib{foo}: cxx{win32-utility}: include = ($cxx.targe.class == 'windows')
exe{bar}: libs{plugin}: include = adhoc

1 files changed, 14 insertions, 0 deletions

diff --git a/build2/variable.ixx b/build2/variable.ixx
index 953bc86..991556a 100644
--- a/build2/variable.ixx
+++ b/build2/variable.ixx
@@ -201,6 +201,20 @@ namespace build2
 
   template <typename T>
   inline T
+  cast_default (const value& v, const T& d)
+  {
+    return v ? cast<T> (v) : d;
+  }
+
+  template <typename T>
+  inline T
+  cast_default (const lookup& l, const T& d)
+  {
+    return l ? cast<T> (l) : d;
+  }
+
+  template <typename T>
+  inline T
   cast_false (const value& v)
   {
     return v && cast<T> (v);