Supply declaration of fdatasync if missing

OS X has the function but doesn't have a declaration for it.

configure.ac

@@ -560,6 +560,7 @@ dnl Checks for library functions.
 AC_CHECK_FUNCS(putenv)
 AC_CHECK_FUNCS(mempcpy)
 AC_CHECK_FUNCS(fdatasync)
+AC_CHECK_DECLS(fdatasync, [], [], [#include <unistd.h>])
 AC_CHECK_FUNCS(lutimes)
 AC_CHECK_FUNCS(mergesort)
 AC_CHECK_FUNCS(getauxval)

system.h

@@ -80,6 +80,10 @@ char * stpncpy(char * dest, const char * src, size_t n);
 #endif
 #endif
 
+#if defined(HAVE_FDATASYNC) && !HAVE_DECL_FDATASYNC
+extern int fdatasync(int fildes);
+#endif
+
 #include "rpmio/rpmutil.h"
 /* compatibility macros to avoid a mass-renaming all over the codebase */
 #define xmalloc(_size) rmalloc((_size))