Don't require <grp.h>

This is only used in the overlay test and Windows does not provide it.
Signed-off-by: Guillem Jover <guillem@hadrons.org>

configure.ac

@@ -59,7 +59,7 @@ AS_CASE([$host_os],
 )
 
 # Checks for header files.
-AC_CHECK_HEADERS([sys/ndir.h sys/dir.h ndir.h dirent.h])
+AC_CHECK_HEADERS([sys/ndir.h sys/dir.h ndir.h dirent.h grp.h])
 
 # Checks for typedefs, structures, and compiler characteristics.
 AC_C_INLINE

test/overlay.c

@@ -28,7 +28,9 @@
  * other headers through magic macros, to check that the overlay is working
  * properly. */
 #include <errno.h>
+#ifdef HAVE_GRP_H
 #include <grp.h>
+#endif
 #include <stdint.h>
 
 /* Include libbsd overlayed headers that might get partially included. */