Implement {c,cxx}.guess modules

These can be loaded before {c,cxx} to guess the compiler. Based on this
information we can then choose the standard, experimental features, etc.
For example:

using cxx.guess

if ($cxx.id == 'clang')
  cxx.features.modules = false

cxx.std = experimental

using cxx

1 files changed, 9 insertions, 0 deletions

diff --git a/build2/c/init.hxx b/build2/c/init.hxx
index e18a5df..5fd7c84 100644
--- a/build2/c/init.hxx
+++ b/build2/c/init.hxx
@@ -15,6 +15,15 @@ namespace build2
   namespace c
   {
     bool
+    guess_init (scope&,
+                scope&,
+                const location&,
+                unique_ptr<module_base>&,
+                bool,
+                bool,
+                const variable_map&);
+
+    bool
     config_init (scope&,
                  scope&,
                  const location&,