Relax substitution requirements, add alternative symbol in version .in support

Give this (legacy) version.h.in:

Can now do:

h{version}: in{version} file{$src_root/manifest}
h{version}: in.symbol = '@'
h{version}: FOO = $project.version

1 files changed, 2 insertions, 0 deletions

diff --git a/build2/version/module.hxx b/build2/version/module.hxx
index 45b1a47..4f9f66d 100644
--- a/build2/version/module.hxx
+++ b/build2/version/module.hxx
@@ -27,6 +27,8 @@ namespace build2
       butl::standard_version version;
       dependency_constraints dependencies;
 
+      const variable* in_symbol = nullptr;
+
       module (butl::standard_version v, dependency_constraints d)
           : version (move (v)), dependencies (move (d)) {}
     };