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, 9 insertions, 1 deletions

diff --git a/build/utility b/build/utility
index 404b33a..6769f95 100644
--- a/build/utility
+++ b/build/utility
@@ -7,7 +7,7 @@
 
 #include <tuple>
 #include <string>
-#include <utility>       // move()
+#include <utility>       // move(), make_pair()
 #include <cassert>       // assert()
 #include <exception>
 #include <unordered_set>
@@ -17,6 +17,7 @@
 namespace build
 {
   using std::move;
+  using std::make_pair;
 
   // Empty string and path.
   //
@@ -24,6 +25,13 @@ namespace build
   extern const path empty_path;
   extern const dir_path empty_dir_path;
 
+  // Parse version string in the X.Y.Z[-{a|b}N] to a version integer in the
+  // AABBCCDD form as describe in <build/version>. Throw invalid_argument
+  // if the passed string is not a valid version.
+  //
+  unsigned int
+  to_version (const string&);
+
   // Call a function if there is an exception.
   //