meson cmake msvc cmd line option detection is wrong

[Neumann-A]

meson/mesonbuild/cmake/toolchain.py

Lines 205 to 209 in 01420bf

	def is_cmdline_option(compiler: 'Compiler', arg: str) -> bool:
	if compiler.get_argument_syntax() == 'msvc':
	return arg.startswith('/')
	else:
	return arg.startswith('-')

This detection is wrong and leads to stupid stuff like:

set(CMAKE_SIZEOF_VOID_P "8")
set(CMAKE_C_COMPILER_LAUNCHER "C:/Program Files/LLVM/bin/clang-cl.exe")
set(CMAKE_C_COMPILER "-DWIN32" <more flags>)

MSVC is totally fine with - options