Enable CERES_HAS_POSIX_BESSEL_FUNCTIONS on Apple platforms

Commit 8b88a9ab49 guarded CERES_HAS_POSIX_BESSEL_FUNCTIONS in port.h with the glibc feature-test macros _SVID_SOURCE, _BSD_SOURCE, and _XOPEN_SOURCE. Because Apple libc++ does not implement C++17 special math functions and Darwin's <math.h> exposes j0, j1, and jn under __DARWIN_C_LEVEL >= 199506L rather than glibc feature-test macros, BesselJ0, BesselJ1, BesselJn, and TEST(Jet, Bessel) were silently disabled on macOS and iOS.

Add __DARWIN_C_LEVEL >= 199506L and _DEFAULT_SOURCE (which replaced _BSD_SOURCE and _SVID_SOURCE in glibc 2.19) to the preprocessor check for CERES_HAS_POSIX_BESSEL_FUNCTIONS.

Change-Id: I17a56983ae0b549346eaf3b5b3a993a6fcc4a3c3