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, 2 insertions, 0 deletions

diff --git a/libbuild2/variable.cxx b/libbuild2/variable.cxx
index 40eeccd..6f9f0fb 100644
--- a/libbuild2/variable.cxx
+++ b/libbuild2/variable.cxx
@@ -3292,6 +3292,8 @@ namespace build2
   template struct LIBBUILD2_DEFEXPORT
   value_traits<vector<pair<string, optional<bool>>>>;
 
+  template struct LIBBUILD2_DEFEXPORT value_traits<set<string>>;
+
   template struct LIBBUILD2_DEFEXPORT
   value_traits<map<string, string>>;