Detect ZLIB using find module

[fbergmann]

Description

This change detects ZLIB using the included Find Module.

Motivation and Context

This solves the problem, that currently conda packages dont build because zlib cannot be found, even though it is compiled and present from the dependencies package.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Change in documentation

Checklist:

• I have updated all documentation necessary.
• I have checked spelling in (new) comments.

Testing

• Testing is done automatically and codecov shows test coverage
• This cannot be tested automatically
  while this is not automatically tested, you can easily see that with this change the variable is picked up from the defined LIBSBML_DEPENDENCY_DIR where before it would have picked up from the system (or not at all)