Rather than issuing echo "cmd..." then cmd... itself, we enable shell trace
facility via set -x, which is POSIX shell standard command and is compatible
with all UNIX shells.
This effectively cuts build.sh size twice, since we don't need to double stuff
there, also making it human readable.
We are also setting PS4 (trace prefix) to none, instead final echo "SUCCESS",
we issue : SUCCESS command which outputs its contents in trace.
See #18 on csources. Basically, some Windows system have gcc as a gcc.bat file. This isn't an issue unless you're calling from another batch file (like here). The call chain will follow to gcc.bat and end there, meaning only one command is processed. Using "CALL" before the other batch command will keep the calls in the main batch file after completion, meaning the compilation will actually succeed this way [see technet](https://technet.microsoft.com/en-us/library/bb490873.aspx). Otherwise you have to hope there is a gcc.exe somewhere instead
Motivation
----------
Currenly tarballs for linux created manually. ZIP files are not
suitable, because they do not preserve unix file permissions.
Modification
------------
Implement 'koch xz' command to produce dist tarball in xz format. Also
the same command implemented in niminst.nim.
Result
------
Now it is easy to create source distribution for git stapshot and use it
further in packaging scripts.
- Niminst has a new -m/--main option to set the main file, by default
ini-file with .nim extension (old behaviour)
- Koch uses this to pass --main:compiler/nim.nim
- Fix includes/links of website.ini
