Add support for configuration exporting and importing

The new config.export variable specifies the alternative file to write the
configuration to as part of the configure meta-operation. For example:

$ b configure: proj/ config.export=proj-config.build

The config.export value "applies" only to the projects on whose root scope it
is specified or if it is a global override (the latter is a bit iffy but we
allow it, for example, to dump everything to stdout). This means that in order
to save a subproject's configuration we will have to use a scope-specific
override (since the default will apply to the outermost amalgamation). For
example:

$ b configure: subproj/ subproj/config.export=.../subproj-config.build

This could be somewhat unnatural but then it will be the amalgamation whose
configuration we normally want to export.

The new config.import variable specifies additional configuration files to be
loaded after the project's default config.build, if any. For example:

$ b create: cfg/,cc config.import=my-config.build

Similar to config.export, the config.import value "applies" only to the
project on whose root scope it is specified or if it is a global override.
This allows the use of the standard override "positioning" machinery (i.e.,
where the override applies) to decide where the extra configuration files are
loaded. The resulting semantics is quite natural and consistent with command
line variable overrides, for example:

$ b   config.import=.../config.build  # outermost amalgamation
$ b ./config.import=.../config.build  # this project
$ b  !config.import=.../config.build  # every project

Both config.export and config.import recognize the special `-` file name as an
instruction to write/read to/from stdout/stdin, respectively. For example:

$ b configure: src-prj/ config.export=- | b configure: dst-prj/ config.import=-

1 files changed, 71 insertions, 24 deletions

diff --git a/libbuild2/file.cxx b/libbuild2/file.cxx
index 603943c..b55d576 100644
--- a/libbuild2/file.cxx
+++ b/libbuild2/file.cxx
@@ -4,8 +4,6 @@
 
 #include <libbuild2/file.hxx>
 
-#include <iostream> // cin
-
 #include <libbuild2/scope.hxx>
 #include <libbuild2/target.hxx>
 #include <libbuild2/context.hxx>
@@ -161,27 +159,39 @@ namespace build2
   }
 
   static void
-  source (scope& root, scope& base, const path& bf, bool boot)
+  source (scope& root, scope& base, lexer& l, bool boot)
   {
     tracer trace ("source");
 
+    const path& bf (l.name ());
+
     try
     {
-      bool sin (bf.string () == "-");
-
-      ifdstream ifs;
-
-      if (!sin)
-        ifs.open (bf);
-      else
-        cin.exceptions (ifdstream::failbit | ifdstream::badbit);
-
-      istream& is (sin ? cin : ifs);
-
       l5 ([&]{trace << "sourcing " << bf;});
 
       parser p (root.ctx, boot);
-      p.parse_buildfile (is, bf, root, base);
+      p.parse_buildfile (l, root, base);
+    }
+    catch (const io_error& e)
+    {
+      fail << "unable to read buildfile " << bf << ": " << e;
+    }
+  }
+
+  static void
+  source (scope& root, scope& base, istream& is, const path& bf, bool boot)
+  {
+    lexer l (is, bf);
+    source (root, base, l, boot);
+  }
+
+  static void
+  source (scope& root, scope& base, const path& bf, bool boot)
+  {
+    try
+    {
+      ifdstream ifs;
+      return source (root, base, open_file_or_stdin (bf, ifs), bf, boot);
     }
     catch (const io_error& e)
     {
@@ -195,6 +205,18 @@ namespace build2
     source (root, base, bf, false);
   }
 
+  void
+  source (scope& root, scope& base, istream& is, const path& bf)
+  {
+    source (root, base, is, bf, false);
+  }
+
+  void
+  source (scope& root, scope& base, lexer& l)
+  {
+    source (root, base, l, false);
+  }
+
   bool
   source_once (scope& root, scope& base, const path& bf, scope& once)
   {
@@ -206,7 +228,7 @@ namespace build2
       return false;
     }
 
-    source (root, base, bf);
+    source (root, base, bf, false);
     return true;
   }
 
@@ -498,18 +520,17 @@ namespace build2
   }
 
   pair<value, bool>
-  extract_variable (context& ctx, const path& bf, const variable& var)
+  extract_variable (context& ctx, lexer& l, const variable& var)
   {
+    const path& bf (l.name ());
+
     try
     {
-      ifdstream ifs (bf);
-
-      lexer lex (ifs, bf);
-      token t (lex.next ());
+      token t (l.next ());
       token_type tt;
 
       if (t.type != token_type::word || t.value != var.name ||
-          ((tt = lex.next ().type) != token_type::assign &&
+          ((tt = l.next ().type) != token_type::assign &&
            tt != token_type::prepend &&
            tt != token_type::append))
       {
@@ -518,7 +539,7 @@ namespace build2
 
       parser p (ctx);
       temp_scope tmp (ctx.global_scope.rw ());
-      p.parse_variable (lex, tmp, var, tt);
+      p.parse_variable (l, tmp, var, tt);
 
       value* v (tmp.vars.find_to_modify (var).first);
       assert (v != nullptr);
@@ -533,6 +554,32 @@ namespace build2
     }
   }
 
+  pair<value, bool>
+  extract_variable (context& ctx,
+                    istream& is,
+                    const path& bf,
+                    const variable& var)
+  {
+    lexer l (is, bf);
+    return extract_variable (ctx, l, var);
+  }
+
+  pair<value, bool>
+  extract_variable (context& ctx, const path& bf, const variable& var)
+  {
+    try
+    {
+      ifdstream ifs (bf);
+      return extract_variable (ctx, ifs, bf, var);
+    }
+    catch (const io_error& e)
+    {
+      fail << "unable to read buildfile " << bf << ": " << e << endf;
+    }
+  }
+
+
+
   // Extract the project name from bootstrap.build.
   //
   static project_name
@@ -745,7 +792,7 @@ namespace build2
         // root scope multiple time.
         //
         if (rs.buildfiles.insert (f).second)
-          source (rs, rs, f, true);
+          source (rs, rs, f, true /* boot */);
         else
           l5 ([&]{trace << "skipping already sourced " << f;});