The CMake feature-detection code used check_symbol_exists(), which
invokes the C compiler. However, some glibc versions don't expose the
fdatasync() declaration when compiled with -std=c11, but do expose it
when compiled with -std=c++11. This most likely comes down to how
_POSIX_SOURCE is defined -- it needs to be >= 201112L for <unistd.h> to
expose fdatasync().
This CL switches to check_cxx_symbol_exists(), which uses the C++
compiler. Asides from fixing the problem above, this is the right thing
to do, because we use <unistd.h> in env_posix.cc, which is compiled with
the C++ compiler.
This CL also fixes a previously introduced inconsistency, where the
macro indicating the fdatasync() feature detection result was referred
to as HAVE_FDATASYNC and HAVE_FUNC_FDATASYNC. The former appears to be
used in other libraries, so this CL switches all our references to
HAVE_FDATASYNC.
Fixes https://github.com/google/leveldb/issues/629
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=228392612
C++11 requires <atomic>. This lets us remove the header detection
(LEVELDB_ATOMIC_PRESENT) and simplify port/atomic_pointer.h.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=189919098
