sparse: check if floatn-common.h is available.

This skip including floatn-common.h if it's not available since it
was introduced in glibc 2.27 and OVS doesn't not actually require
that to work with previous glibc version.

Fixes: 07aec2a sparse: Support newer GCC/glibc versions.
Signed-off-by: Flavio Leitner <fbl@sysclose.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>

configure.ac

@@ -110,7 +110,7 @@ AC_CHECK_MEMBERS([struct sockaddr_in6.sin6_scope_id], [], [],
 #include <netinet/in.h>]])
 AC_CHECK_FUNCS([mlockall strnlen getloadavg statvfs getmntent_r sendmmsg clock_gettime])
 AC_CHECK_HEADERS([mntent.h sys/statvfs.h linux/types.h linux/if_ether.h])
-AC_CHECK_HEADERS([linux/net_namespace.h stdatomic.h])
+AC_CHECK_HEADERS([linux/net_namespace.h stdatomic.h bits/floatn-common.h])
 AC_CHECK_HEADERS([net/if_mib.h], [], [], [[#include <sys/types.h>
 #include <net/if.h>]])
 

include/sparse/bits/floatn.h

@@ -27,6 +27,9 @@
 #define __HAVE_FLOAT128 0
 #define __HAVE_FLOAT64X 0
 
+#ifdef HAVE_BITS_FLOATN_COMMON_H
+/* Introduced in glibc 2.27 */
 #include <bits/floatn-common.h>
+#endif
 
 #endif /* <bits/floatn.h> for sparse */