Add support for relative to base scope command line variable overrides

Currently, if we say:

$ b dir/ ./foo=bar

The scope the foo=bar is set on is relative to CWD, not dir/. While this
may seem wrong at first, this is the least surprising behavior when we
take into account that there can be multiple dir/'s.

Sometimes, however, we do want the override directory to be treated relative
to (every) target's base scope that we are building. To support this we are
extending the '.' and '..' special directory names (which are still resolved
relative to CWD) with '...', which means "relative to the base scope of every
target in the buildspec". For example:

$ b dir/ .../foo=bar

Is equivalent to:

$ b dir/ dir/foo=bar

And:

$ b liba/ libb/ .../tests/foo=bar

Is equivalent to:

$ b liba/ libb/ liba/tests/foo=bar libb/tests/foo=bar

1 files changed, 2 insertions, 1 deletions

diff --git a/build2/scope.hxx b/build2/scope.hxx
index 7929bce..a0c96f0 100644
--- a/build2/scope.hxx
+++ b/build2/scope.hxx
@@ -352,7 +352,7 @@ namespace build2
     // global scope with empty key.
     //
     iterator
-    insert (const dir_path&, bool root);
+    insert (const dir_path&, bool root = false);
 
     // Find the most qualified scope that encompasses this path.
     //
@@ -396,6 +396,7 @@ namespace build2
 
     // Entities that can access bypassing the lock proof.
     //
+    friend int main (int, char*[]);
     friend variable_overrides reset (const strings&);
 
     scope&