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, 2 insertions, 0 deletions

diff --git a/build2/b.cxx b/build2/b.cxx
index 95bf876..0520bd8 100644
--- a/build2/b.cxx
+++ b/build2/b.cxx
@@ -333,9 +333,11 @@ main (int argc, char* argv[])
       bm["cc.config"] = mf {nullptr, &cc::config_init};
       bm["cc"] = mf {nullptr, &cc::init};
 
+      bm["c.guess"] = mf {nullptr, &c::guess_init};
       bm["c.config"] = mf {nullptr, &c::config_init};
       bm["c"] = mf {nullptr, &c::init};
 
+      bm["cxx.guess"] = mf {nullptr, &cxx::guess_init};
       bm["cxx.config"] = mf {nullptr, &cxx::config_init};
       bm["cxx"] = mf {nullptr, &cxx::init};