remove some duplicate CMake code

I did see a lot of duplication in src/libelektra/CMakeLists.txt just because there are two different target names.

This commit does the following:
-) Set the target name as a variable (depending on build type)
-) Set the type of library to be built (static or shared, depending on build type)
-) use the variable with the target name in the rest of the CMake buildscript
If there are usecases to build both, this change of course isn't compatible with this use-case. I thought of using loops or so, but that wouldn't make the CMake code much simpler. So I'm closing this pull request.