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, 31 insertions, 0 deletions

diff --git a/build2/prerequisite.ixx b/build2/prerequisite.ixx
new file mode 100644
index 0000000..aeec6b4
--- /dev/null
+++ b/build2/prerequisite.ixx
@@ -0,0 +1,31 @@
+// file      : build2/prerequisite.ixx -*- C++ -*-
+// copyright : Copyright (c) 2014-2018 Code Synthesis Ltd
+// license   : MIT; see accompanying LICENSE file
+namespace build2
+{
+  include_type
+  include_impl (action,
+                const target&,
+                const string&,
+                const prerequisite&,
+                const target*);
+
+  extern const variable* var_include; // context.cxx
+
+  inline include_type
+  include (action a, const target& t, const prerequisite& p, const target* m)
+  {
+    // Most of the time this variable will not be specified, so let's optimize
+    // for that.
+    //
+    if (p.vars.empty ())
+      return true;
+
+    const string* v (cast_null<string> (p.vars[var_include]));
+
+    if (v == nullptr)
+      return true;
+
+    return include_impl (a, t, *v, p, m);
+  }
+}