mk: Compile C code on MSVC with /MD

mk: Compile C code on MSVC with /MD

On MSVC there are two ways that the CRT can be linked, either statically or
dynamically. Each object file produced by the compiler is compiled against
msvcrt (a dll) or libcmt (a static library). When the linker is dealing with
more than one object file, it requires that all object files link to the same
CRT, or else the linker will spit out some errors.

For now, compile code with `-MD` as it seems to appear more often in C libraries
so we'll stick with the same trend.