Add string_set buildfile value type

This exposes the std::set<std::string> type to buildfiles.

New functions:

  $size(<string-set>)

Subscript returns true if the value is present and false otherwise (so
it is mapped to std::set::contains()). For example:

set = [string_set] a b c

if ($set[b])
  ...

Note that append (+=) and prepend (=+) have the same semantics
(std::set::insert()). For example:

set = [string_set] a b
set += c b              # a b c
set =+ d b              # a b c d

Example of iteration:

set = [string_set] a b c
for k: $set
  ...

1 files changed, 4 insertions, 2 deletions

diff --git a/libbuild2/functions-string.cxx b/libbuild2/functions-string.cxx
index b332efa..367923f 100644
--- a/libbuild2/functions-string.cxx
+++ b/libbuild2/functions-string.cxx
@@ -119,14 +119,16 @@ namespace build2
     };
 
     // $size(<strings>)
+    // $size(<string-set>)
     // $size(<string-map>)
     // $size(<string>)
     //
-    // First two forms: return the number of elements in the sequence.
+    // First three forms: return the number of elements in the sequence.
     //
-    // Third form: return the number of characters (bytes) in the string.
+    // Fourth form: return the number of characters (bytes) in the string.
     //
     f["size"] += [] (strings v)             {return v.size ();};
+    f["size"] += [] (set<string> v)         {return v.size ();};
     f["size"] += [] (map<string, string> v) {return v.size ();};
     f["size"] += [] (string v)              {return v.size ();};