Mentioning msvcrt.dll sounds to me like your real concern is proprietary windows-specific functions that might not be universally (or "eternally") available, thus breaking your applications if they (msvcrt.dll, etc, https://en.wikipedia.org/wiki/Microsoft_Windows_library_files ) are dependencies. I think you might be happy/content using Posix standard library functions, see, e.g., http://stackoverflow.com/questions/9376837/difference-between-c-standard-library-and-c-posix-library or https://en.wikipedia.org/wiki/C_POSIX_library for which the source code is freely available, e.g., https://www.gnu.org/software/libc/
So maybe what you want to do is determine which windows standard library functions are identical to posix counterparts, and restrict your application code to that subset. It's kind of impossible to do anything non-trivial without using any standard library stuff whatsoever.