Enhance error message for dependency check (#96642)
If python development library is missing when building pytorch from source, cmake will raise the error like:
```
CMake Error at cmake/Dependencies.cmake:1079 (if):
if given arguments:
"VERSION_LESS" "3"
Unknown arguments specified
```
it's quite a misleading information that user would consider it's a syntax error or cmake version problem.
This PR add a check to ensure `PYTHONLIBS_VERSION_STRING` exist before using.
Related #87993
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96642
Approved by: https://github.com/kit1980
