Build against Yocto SDK

Currently building with:

BUILD_TARGET=x86_64-yocto-linux ./install.sh

seems not to use the yocto specified compiler. (The resulting libsnap7.so have the same size for x86_64-yocto-linux and x86_64-linux)

I suppose the problem is, that build/unix/common.mk overwrites CC, CXX and possibly other variables which were set by yocto (source /common/usr/embedded/yocto/sdk/environment-setup-core2-64-ffos-linux)

So we probably need to apply a patch to the sources, before we can build.

Another possible option would be the usage of cmake to build snap7. There is already a community-build CMakeLists.txt available:

https://sourceforge.net/p/snap7/discussion/general/thread/8d18f328e7/#3b5e (to be tested)

That as well would have the advantage to give use a static snap7 library, which would simplify rollout.