Add support for ~host special configuration name in config.import

This is the "default host configuration" that corresponds to how the build
system itself was built. For example:

$ b create: tools/,cc config.import=~host

1 files changed, 13 insertions, 0 deletions

diff --git a/libbuild2/config/host-config.cxx.in b/libbuild2/config/host-config.cxx.in
new file mode 100644
index 0000000..46cd912
--- /dev/null
+++ b/libbuild2/config/host-config.cxx.in
@@ -0,0 +1,13 @@
+// file      : libbuild2/config/host-config.cxx.in -*- C++ -*-
+// copyright : Copyright (c) 2014-2019 Code Synthesis Ltd
+// license   : MIT; see accompanying LICENSE file
+
+namespace build2
+{
+  namespace config
+  {
+    // This is a raw string literal, in case you are unfamiliar.
+    //
+    extern const char host_config[] = R"###($host_config$)###";
+  }
+}