Fix MSVC 14 compilation errors
For _MSC_VER == 1900 these macros are not needed and create problems:
1>C:\Program Files (x86)\Windows Kits\10\Include\10.0.10240.0\ucrt\stdio.h(1925): warning C4005: 'snprintf': macro redefinition (compiling source file libssh2-files\src\mac.c)
1> \win32\libssh2_config.h(27): note: see previous definition of 'snprintf' (compiling source file libssh2-files\src\mac.c)
1>C:\Program Files (x86)\Windows Kits\10\Include\10.0.10240.0\ucrt\stdio.h(1927): fatal error C1189: #error: Macro definition of snprintf conflicts with Standard Library function declaration (compiling source file libssh2-files\src\mac.c)
1 file changed
