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, 2 insertions, 0 deletions

diff --git a/build/types b/build/types
index c119839..2dc9b04 100644
--- a/build/types
+++ b/build/types
@@ -7,6 +7,7 @@
 
 #include <vector>
 #include <string>
+#include <utility>     // pair
 #include <memory>      // unique_ptr, shared_ptr
 #include <functional>  // reference_wrapper
 
@@ -19,6 +20,7 @@ namespace build
 {
   // Commonly-used types.
   //
+  using std::pair;
   using std::string;
   using std::unique_ptr;
   using std::shared_ptr;