Move _POSIX_C_SOURCE check after sys/types.h

If the user hasn't explicitly declared _POSIX_C_SOURCE to the compiler,
then we want to make sure the that system's default feature test macros
are included before checking against them. We could explicitly include
`<features.h>`, but since this is glibc specific, just delay the check
until after including `<sys/types.h>`, which will pull in `<features.h>` as
part of glibc's implementation. See feature_test_macros(7) for details.

[endlessm/eos-sdk#2449]