Support Canadian cross builds on FreeBSD (and other hosts).

Set up the rules to manage the separate host and build setting to
allow a Canadian cross to complete.

Update the scripts to move the build directory and host/build
flags into the defaults so they are common for all build
configurations.

source-builder/sb/check.py

@@ -91,6 +91,7 @@ def _check_paths(name, paths):
                 return True
     return False
 
+
 def host_setup(opts):
     """ Basic sanity check. All executables and directories must exist."""
 
@@ -125,6 +126,10 @@ def host_setup(opts):
     return sane
 
 
+def check_exe(label, exe):
+    return _check_exe(None, label, exe, None)
+
+
 def run():
     import sys
     try: