Add extra support for symbol exporting in modularized projects

Since modules don't see each other's macros, we can use a single, keyword-
like macro for dll-exporting that is managed by the build system (so no
need for an "export" header). For example:

cxx.features.symexport = true

export __symexport void f ();

1 files changed, 3 insertions, 0 deletions

diff --git a/build2/cc/compile.hxx b/build2/cc/compile.hxx
index be4a2e2..ed9c18d 100644
--- a/build2/cc/compile.hxx
+++ b/build2/cc/compile.hxx
@@ -123,6 +123,9 @@ namespace build2
       const char*
       langopt (const match_data&) const;
 
+      void
+      append_symexport_options (cstrings&, const target&) const;
+
     private:
       const string rule_id;
     };