open() flag O_DSYNC isn't on BSD, use O_SYNC

If the detected OS is FreeBSD, tell LMDB to compile with
MDB_DSYNC=O_SYNC instead of the default O_DSYNC, as BSD does not
implement this flag.

external/db_drivers/liblmdb32/CMakeLists.txt

@@ -26,6 +26,10 @@
 # STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
 # THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
+if(FREEBSD)
+  set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -DMDB_DSYNC=O_SYNC")
+endif()
+
 set (lmdb_sources
 mdb.c
 midl.c)

external/db_drivers/liblmdb64/CMakeLists.txt

@@ -26,6 +26,10 @@
 # STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
 # THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
+if(FREEBSD)
+  set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -DMDB_DSYNC=O_SYNC")
+endif()
+
 set (lmdb_sources
 mdb.c
 midl.c)