cmake: make use of waffle_on_* variables

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
readme_msvc: add a notes about msvc builds

 - set msvc build environment
 - cmake arguments
 - ^^ for both win32 and win64 build

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
cmake: silence nonstdc warnings (msvc)

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>