Mxebuild fix

This patch mostly consists of changes to make it easier to install/build dependencies on linux/BSD and get rid of the use of "/usr/local" and LD_LIBRARY_PATH in the instructions. There are also other minor changes for release like putting the version in the About box. These modifications were tested on 16 different linux/bsd systems. 

Changes in cosmetic stuff:

- Acknowledgements in About box hand-recoded in separate html file
- OpenSCAD version is explicitly described in the About Box text
- Help menu has 'H' shortcut

Change in copyright

- Copyright explicitly assigned to C. Wolf + M. Kintel, to prevent Debian issues

Changes in basic build files

- rewrite of Linux/BSD build section of README.md 
- release-common script improved for MXE. Linux has been tested to work as well
- openscad.pro + *pri - minor fixes to eigen, flex, bison for BSD, a few bug fixes
- RPATH used if dependencies are built, to avoid LD_LIBRARY_PATH issues

Changes in build instructions

Automatically try to fetch dependencies using package managers:

    ./scripts/uni-get-dependencies.sh

Automatically check versions and advise user:

    ./scripts/check-dependencies.sh

If there are missing/old dependencies, auto download and build

    source ./scripts/setenv-unibuild.sh
    ./scripts/uni-build-dependencies.sh 

Changes in build dependency script
- build script allows 'opencsg' and 'cgal' as arguments for single build
- it wont re-build already built dependencies
- 'out of tree' dependency builds are possible
- experimental clang support on linux

Platforms tested:

Linux:
ubuntu 12.10 / x86
ubuntu 12.04 / x86
ubuntu 12.04 clang / x86
ubuntu 10.04 / x86
opensuse 12 / x86
mageira 2 / x86
debian 7 / amd64
debian 6 / x86
debian 6 / amd64
debian 5 / amd64
fedora 16 / ppc64
redhat 6.3 / amd64
altlinux simplylinux 6 / x86

Other:
debian hurd 7 / x86
freebsd 9.0 / x86
netbsd 6.0 / amd64