Add support for specifying minimum required build2 version

The syntax is:

using build@0.1.0-a1

The idea is that we will later also use it for modules and 'build' is
a special, the "build system itself" module.

Also fix a problem with peeking and lexer mode switching.

1 files changed, 3 insertions, 0 deletions

diff --git a/build/parser b/build/parser
index f3f88d5..08d587a 100644
--- a/build/parser
+++ b/build/parser
@@ -164,6 +164,9 @@ namespace build
     token_type
     next (token&, token_type&);
 
+    // Be careful with peeking and switching the lexer mode. See keyword()
+    // for more information.
+    //
     token_type
     peek ();