Everyone writes in vendor-specific dialects of C. Both POSIX and Win32 define behaviour that isn't defined in C (such as rules for unaligned pointers), while also undefining behaviour that is defined in C (such as what happens if you call fopen when one of the functions in your program is called "open"). Even on embedded platforms, there's no "main" function in freestanding C - your entry point is a vendor-specific extension.
> Everyone writes in vendor-specific dialects of C.
That's not true. Some people try to write fully spec compliant C programs that would behave the same on every compliant C compiler.
Most projects don't use that flag though, why not?
Nite that if you assume -fwrapv, you're not writing in C anymore, you're using a vendor specific dialect.