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, 22 insertions, 3 deletions

diff --git a/libbuild2/buildfile b/libbuild2/buildfile
index 865bb91..325d54a 100644
--- a/libbuild2/buildfile
+++ b/libbuild2/buildfile
@@ -32,10 +32,29 @@ lib{build2}: cxx{utility-uninstalled}: for_install = false
 # NOTE: remember to update import_modules() in libbuild2/modules.cxx if adding
 #       a new such module.
 #
-for m: config dist install test
-  libul{build2}: $m/{hxx ixx txx cxx}{** -**-options -**.test...}
+libul{build2}: config/{hxx ixx txx cxx}{** -host-config -**.test...} \
+               config/cxx{host-config}
 
-libul{build2}: test/script/{hxx ixx cxx}{builtin-options}
+# This will of course blow up spectacularly if we are cross-compiling. But
+# let's wait and enjoy the fireworks (and get a sense of why would someone
+# need to cross-compile a build system).
+#
+newline = '
+'
+config/cxx{host-config}: config/in{host-config}
+{
+  # Remove comment lines which could be confused by some lesser compilers with
+  # preprocessor directives.
+  #
+  host_config = $regex.replace($config.export(), "#.*$newline", '')
+}
+
+libul{build2}: dist/{hxx ixx txx cxx}{** -**.test...}
+
+libul{build2}: install/{hxx ixx txx cxx}{** -**.test...}
+
+libul{build2}: test/{hxx ixx txx cxx}{** -**-options -**.test...} \
+               test/script/{hxx ixx cxx}{builtin-options}
 
 libul{build2}: $int_libs