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, 1 insertions, 0 deletions

diff --git a/libbuild2/parser.cxx b/libbuild2/parser.cxx
index b68f009..bf806be 100644
--- a/libbuild2/parser.cxx
+++ b/libbuild2/parser.cxx
@@ -6101,6 +6101,7 @@ namespace build2
           if (n[6] == '\0') return &value_traits<string>::value_type;
           if (n[6] == 's' &&
               n[7] == '\0') return &value_traits<strings>::value_type;
+          if (n == "string_set") return &value_traits<set<string>>::value_type;
           if (n == "string_map")
             return &value_traits<map<string,string>>::value_type;
         }