`R CMD check` issues on Windows

[gaborcsardi]

• Line endings in generated (?) source files: #2
• Line endings in generated Makefiles: #4
• Pick up rtools44 (etc.) location from env var instead of hard-coding it #5
• R 4.5.x installation (Rtools45 will be patches, hopefully: https://bugs.r-project.org/show_bug.cgi?id=18869)
• R 4.3.x installation
• R 4.2.x installation
• R 4.1.x installation
• R 4.1.0 installation

[gaborcsardi]

Will fix R <= 4.2.0 on Windows later, if needed.