Define _CRT_SECURE_NO_WARNINGS when compiling argp with MSVC.
argp-parse.c: include unistd.h only if it's available.
argp-helpt.c: removed C dialect and compiler specific keywords from function declarations.
Fix for missing unlocked variants of stdio functions on windows. Just need to probe them, no code modification is needed.
Preparations in build process for compatibility module argp-compat.c