Regarding the libcrypt.m4 problem i mentioned above, I found an bugreport describing the problem:
https://bugs.launchpad.net/debian/+source/libmcrypt/+bug/253991
There is also explained how to overcome this error.
Regarding my other problem:
I did a clean "git clone" and followed the howto exactly. Not it works.
The source-folder I tried to compile first was a modified version which I got from a collegue. He compiled it without problems on his machine, so he gave it to me. It seems, that this first compile created some temporary files, which obviously had some sideeffects on the configure/build process on my machine.
best regards and thanks for reading,
Alex