Also worth noting that WinFlexBison has been packaged for the Chocolatey package manager. Install that and then go:
choco install winflexbison
...which at the time of writing contains Bison 2.7 & Flex 2.6.3.
There is also winflexbison3 which (at the time of writing) has Bison 3.0.4 & Flex 2.6.3.