Fix parser problems in final executable
This MR fixes some problems when trying to use the SymEngine and ExprTk parsers in the final program.
- This fixes the path to a wrong header and linker errors.
- Enables fetching exprtk from the internet by default. This way, the user is not required to have muparser in the system.
- Finally, it adds a command line to know which parsers are actually compiled into the program.