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