Build: Fix the detection of missing CRC32.

Thanks to Vincent Torri.
diff --git a/configure.ac b/configure.ac
index 97dfb27..186a39b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -260,7 +260,7 @@
 	done
 	AC_MSG_RESULT([$enable_checks])
 fi
-if test "x$enable_checks_crc32" = xno ; then
+if test "x$enable_check_crc32" = xno ; then
 	AC_MSG_ERROR([For now, the CRC32 check must always be enabled.])
 fi