Build: Use POSIX.1–2001 (IEEE Std 1003.1-2001)

Define `_POSIX_C_SOURCE` to `200112L`, to indicate we want to use
POSIX.1–2001. Additionally remove incorrect definition of
`POSIX_C_SOURCE` (note the lack of underscore at the beginning).
The intention was probably to define `_POSIX_C_SOURCE` which would be
redundant and previously even conflicting with the other version set
previously.
For the has_function check, use the correct value of `200112L`.