Fix OpenMP support for cross-platform compatibility

[Arshammik]

The package currently uses OpenMP for parallelization with the following configuration in src/Makevars:

CXX_STD = CXX14
PKG_CXXFLAGS += -O3 -march=native -pipe -fopenmp
PKG_LIBS     += -fopenmp $(LAPACK_LIBS) $(BLAS_LIBS) $(FLIBS)

This works fine on Linux but fails to compile on macOS with the following error:

clang++: error: unsupported option '-fopenmp'

This happens because Apple's default Clang compiler doesn't support OpenMP. Additionally, R CMD check gives warnings about GNU extensions.

Proposed Solution

Create platform-specific Makevars files and a configure script to automatically handle the differences between operating systems:

1. Keep current settings for Linux in src/Makevars
2. Create src/Makevars.win for Windows
3. Create src/Makevars.mac for macOS without OpenMP flags
4. Add a configure script that detects macOS and uses the appropriate Makevars file
5. Add documentation for users who want OpenMP support on macOS
6. Add a function to check if OpenMP is enabled at runtime