Windows-Friendly Makefile

[evanperrygrove-sibelinc]

The Makefile created by rdm init could be Windows-friendly with minimal changes. I don't have the time to put together a proper PR right now. However, I can provide the key piece that someone would need to add:

ifeq ($(OS),Windows_NT)
MKDIR_P := powershell New-Item -ItemType Directory -Force -ea 0
RM_RF := powershell Remove-Item -Recurse -Force -ea 0
else
MKDIR_P := mkdir -p
RM_RF := rm -rf
endif

(edited to use PowerShell commands, since PowerShell handles forward slashes correctly)