define _DEFAULT_SOURCE on non-windows environments to prevent feature test macros from removing functions like clock_gettime()

[irremnent]

When using a C compiler like gcc or clang, specifying a standard such as -std=c11 will define special macros that will cause the compiler to strip out certain definitions from header files--namely time.h. This will cause nob to fail to compile due to implicit declaration of functions like clock_gettime(), nanosleep(), and lstat(). To prevent this, _DEFAULT_SOURCE can be defined before including any header files.

I am unsure if this is an issue on windows systems.