fix some sound bug

in some cases, TraceLine may shorten the shotline to length zero, which
causes addshotline() to produces illegal coordinates for sound effects
report openal errors with more detail

(to make debugging further sound issues easier)
position checking may be reduced again in a while...
Added AC icon to SDL window (fixes #29)

As zeSKB reported, there was no icon for the game window.
This should now be fixed. We might want to make it transparent in the future,
but we would have to make sure it will be supported by any system.
transparency options for console and votes (resolves #45)
makefile with easier customisation

This makefile allows changing the compiler to g++ or requesting debug
binaries without editing the makefile.

The makefile sets default values for CXX and ACDEBUG, then reads values
from the file Makefile_local (if it exists). A commandline parameter
overrules both.

Makefile_local is not part of the repository, so it can be changed without
git caring (or nagging) about it. It is listed in .gitignore.

To switch to g++, create a file "Makefile_local" with the content
"CXX=g++".

To compile debug binaries, type "make clean install ACDEBUG=yes".

When g++ is used, "fast-math" is now enabled.