build: Use AC_HEADER_MAJOR to find device macros

Depending on the OS/libc, device macros are defined in different
headers. This change ensures we include the right one.

sys/types.h - BSD
sys/mkdev.h - Sun
sys/sysmacros.h - glibc (Linux)

glibc currently pulls sys/sysmacros.h into sys/types.h, but this may
change in a future release.

https://sourceware.org/ml/libc-alpha/2015-11/msg00253.html

grub-core/osdep/linux/getroot.c

@@ -35,6 +35,12 @@
 #include <limits.h>
 #endif
 
+#if defined(MAJOR_IN_MKDEV)
+#include <sys/mkdev.h>
+#elif defined(MAJOR_IN_SYSMACROS)
+#include <sys/sysmacros.h>
+#endif
+
 #include <grub/types.h>
 #include <sys/ioctl.h>         /* ioctl */
 #include <sys/mount.h>