host and target
~~~~~~~~~~~~~~~

Scconfig supports crosscompiling by separaating tests done for the host
platform and for the target platform. The host platform is the one that
hosts compilation while the target platform is where the final binaries
produced by the project embedding scconfig will run.

The property tree is divided up to root directories. One such directory
is /host/ and another is /target/. If scconfig is not running in cross mode
so target is the same as host, then /target/ is a symlink to /host/.

In any case, there are tests done for the host system first. These are
the tests needed for the compile infrastructure of the project to work.
For example if the project needs to compile tools that are used during
compilation, these tools will run on the host computer, whatever the
target computer is. Any utility (including the compiler) that is needed to
get those tools compiled and run must be detected for the host.

Scconfig will first run tests for the host system (calling user
provided callback hook_detect_host()) and then tests for the target
system (using hook_detect_target()). When working for the target system,
scconfig will use the specified emulator to run the test programs or
if no emulator is available it will simply assume the test programs
worked if they compiled. 

If the host system is the target system, some tests may overlap. For example
for the local tools cc/cc is needed on the host and for the final compilation
cc/cc is needed for the target. In short, this means both /host/cc/cc and
/target/cc/cc are needed. If scconfig is running in non-cross mode, that means
/target is linked to /host, thus the two cc/cc nodes are the same. When
cc/cc detection for host runs, it will find the C compiler and will store it
in the tree and next when the same test would run for target, the result is
already there so the test is skipped. This way overlapping requirements on
host and target systems won't result in duplicate tests in case we
are not doing cross compilation.