va_copy unavailable when using stdarg.h with -ansi on Linux

[memreflect]

(This is related to #3.)

Despite va_copy() being required by POSIX.1-2001, it may be the compiler that defines it rather than the C library headers.
That is, glibc should be the one defining va_copy() since it's the one interfacing with the Linux kernel to implement POSIX functionality, but instead, it leaves things up to the gcc compiler, which only concerns itself with the ISO C standard and doesn't care about POSIX stuff.
The result of this disconnect is the linker used by gcc on Linux being unable to resolve va_copy() even when compiling with -D_POSIX_C_SOURCE=200112L.

I'm not sure how best to handle this, but i should note that even when va_copy() is undefined by gcc, __va_copy() is still available.
Perhaps it's worth defining va_copy() as __va_copy() when using <stdarg.h> as well, if it's not already available?