fdatasync is not documented on Snow Leopard and is not in unistd.h

CMake finds it as it is expored by system libraries, though attempts to
use it will result bring a warning. Fix by using CHECK_SYMBOL_EXISTS
instead of CHECK_FUNCTION_EXISTS

configure.cmake

@@ -21,6 +21,7 @@ INCLUDE (CheckLibraryExists)
 INCLUDE (CheckFunctionExists)
 INCLUDE (CheckCCompilerFlag)
 INCLUDE (CheckCSourceRuns)
+INCLUDE (CheckSymbolExists)
 
 
 # Sometimes it is handy to know if PIC option
@@ -521,7 +522,7 @@ ENDIF()
 CHECK_FUNCTION_EXISTS_UNIX (fchmod HAVE_FCHMOD)
 CHECK_FUNCTION_EXISTS_UNIX (fcntl HAVE_FCNTL)
 CHECK_FUNCTION_EXISTS_UNIX (fconvert HAVE_FCONVERT)
-CHECK_FUNCTION_EXISTS_UNIX (fdatasync HAVE_FDATASYNC)
+CHECK_SYMBOL_EXISTS_UNIX(fdatasync "unistd.h" HAVE_FDATASYNC)
 CHECK_FUNCTION_EXISTS_UNIX (fesetround HAVE_FESETROUND)
 CHECK_FUNCTION_EXISTS_UNIX (fpsetmask HAVE_FPSETMASK)
 CHECK_FUNCTION_EXISTS_UNIX (fseeko HAVE_FSEEKO)
@@ -657,7 +658,7 @@ CHECK_FUNCTION_EXISTS_UNIX(rdtscll HAVE_RDTSCLL)
 #
 # Tests for symbols
 #
-INCLUDE (CheckSymbolExists)
+
 CHECK_SYMBOL_EXISTS_UNIX(sys_errlist "stdio.h" HAVE_SYS_ERRLIST)
 CHECK_SYMBOL_EXISTS_UNIX(madvise "sys/mman.h" HAVE_DECL_MADVISE)
 CHECK_SYMBOL_EXISTS_UNIX(tzname "time.h" HAVE_TZNAME)