Fix fdatasync() feature detection in opensource build.

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

1 files changed, 7 insertions, 2 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 36d6cbd..57a0c74 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -30,8 +30,13 @@ check_library_exists(crc32c crc32c_value "" HAVE_CRC32C)
 check_library_exists(snappy snappy_compress "" HAVE_SNAPPY)
 check_library_exists(tcmalloc malloc "" HAVE_TCMALLOC)
 
-include(CheckSymbolExists)
-check_symbol_exists(fdatasync "unistd.h" HAVE_FDATASYNC)
+include(CheckCXXSymbolExists)
+# Using check_cxx_symbol_exists() instead of check_c_symbol_exists() because
+# we're including the header from C++, and feature detection should use the same
+# compiler language that the project will use later. Principles aside, some
+# versions of do not expose fdatasync() in <unistd.h> in standard C mode
+# (-std=c11), but do expose the function in standard C++ mode (-std=c++11).
+check_cxx_symbol_exists(fdatasync "unistd.h" HAVE_FDATASYNC)
 
 include(CheckCXXSourceCompiles)