Allow release builds with asserts on windows (#882)
Allow release builds with asserts on windows (#882)

The posix build enables assertions on release builds with
ADD_NONDEBUG_COMPILE_FLAG("-UNDEBUG")
but the windows build does not. This PR adds /UNDEBUG to the release
build flags but has the additional complication that /DNDEBUG is added
to CFLAGS by CMake by default, and MSVC will warn about having /UFOO
after /DFOO on the command line. So we scrub DNDEBUG from CFLAGS
as well.

Additional windows build cleanups:

Disable conversion/truncation warnings
Canonicalize flag style to use slashes everywhere instead of mixing
MSVC uses /Od (not /O0) to disable optimization
yurydelendik/binaryen,WebAssembly/binaryen,WebAssembly/binaryen,WebAssembly/binaryen,WebAssembly/binaryen,yurydelendik/binaryen,yurydelendik/binaryen,yurydelendik/binaryen,yurydelendik/binaryen,WebAssembly/binaryen