Query optional build features through the C API

[paulromano]

Description

Another PR moving us toward proper Windows support along the same lines as #4062 (avoiding need for OPENMC_API). This PR replaces the exported DAGMC, libMesh, strict floating-point, and UWUW boolean globals with a single openmc_get_feature_enabled C API function. The Python API exposes this as openmc.lib.feature_enabled, and all tests now query features directly at their point of use.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[GuySten]

Looks good to me.