[R] Fix winbuilder check warnings.

[trivialfis]

• Move files out of the project directory.
• Improve cache prefix handling for external memory.
• Fix compiler warnings about function prototype.
• Restrict threads in examples.

[trivialfis]

Found the following significant warnings:
  init.c:61:1: warning: function declaration isn't a prototype [-Wstrict-prototypes]
  init.c:68:1: warning: function declaration isn't a prototype [-Wstrict-prototypes]
See 'd:/RCompile/CRANguest/R-oldrelease/xgboost.Rcheck/00install.out' for details

[Copilot]

Pull request overview

This PR addresses R package check warnings by making several corrections across the codebase. The changes include proper handling of cache directory paths, updating C function declarations to use proper parameter specifications, relocating configuration headers to the correct directory, and adding thread control directives in R documentation examples.

Key changes:

• Enhanced cache path handling to properly detect and handle directory inputs in the MakeCachePrefix function
• Fixed C function declarations to use (void) instead of () for parameterless functions, which is the correct C standard practice
• Relocated config.h from the R package root to the src/ subdirectory for proper R package structure

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/data/sparse_page_source.cc	Added directory detection logic to cache prefix handling and updated copyright year
tests/cpp/helpers.cc	Updated test to use MakeCachePrefix wrapper for consistency with production code
ops/script/test_r_package.py	Added removal of bootstrap.R file during R package creation
R-package/src/init.c	Fixed function declarations to use (void) instead of () for parameterless functions
R-package/src/config.h.in	Updated generated file comment to reflect correct path
R-package/configure.ac	Changed config header path from config.h to src/config.h
R-package/configure	Regenerated configure script with updated config header path
R-package/man/predict.xgb.Booster.Rd	Added thread control directive for R CMD check
R-package/man/a-compatibility-note-for-saveRDS-save.Rd	Added thread control directive for R CMD check
R-package/R/xgb.Booster.R	Added thread control directive in example documentation
R-package/R/utils.R	Added thread control directive in example documentation

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