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 someone who builds the library sets both should both be built? This would of course change that behaviour but that is hopefully not a common or much wanted use case.