configure: add check for bswap_64

include/f2fs_fs.h checks the HAVE_BSWAP_64 conidtional, but configure
nevers checks for it.

Add a check that the function is indeed declared, and fix the variable
name to match the naming scheme of autoconf (and not those of WAF like
is used by samba), and adapt the check as suggested in the autoconf
manual.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/configure.ac b/configure.ac
index 7cfd9b4..ae451b8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -72,6 +72,9 @@
 	memset
 ])
 
+AS_IF([test "$ac_cv_header_byteswap_h" = "yes"],
+      [AC_CHECK_DECLS([bswap_64],,,[#include <byteswap.h>])])
+
 # Install directories
 AC_PREFIX_DEFAULT([/usr])
 AC_SUBST([sbindir], [/sbin])
diff --git a/include/f2fs_fs.h b/include/f2fs_fs.h
index 0c3ba04..4dc2426 100644
--- a/include/f2fs_fs.h
+++ b/include/f2fs_fs.h
@@ -63,7 +63,7 @@
 }
 #endif /* !HAVE_BYTESWAP_H */
 
-#if !HAVE_BSWAP_64
+#if defined HAVE_DECL_BSWAP_64 && !HAVE_DECL_BSWAP_64
 /**
  * bswap_64 - reverse bytes in a uint64_t value.
  * @val: value whose bytes to swap.