patch 8.2.0315: build failure on HP-UX system

Problem:    Build failure on HP-UX system.
Solution:   Use LONG_LONG_MIN instead of LLONG_MIN.  Add type casts for switch
            statement. (John Marriott)
diff --git a/src/structs.h b/src/structs.h
index 7830e64..e68c1f9 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -1279,9 +1279,15 @@
 #else
   typedef long long		varnumber_T;
   typedef unsigned long long	uvarnumber_T;
-# define VARNUM_MIN		LLONG_MIN
-# define VARNUM_MAX		LLONG_MAX
-# define UVARNUM_MAX		ULLONG_MAX
+# ifdef LLONG_MIN
+#  define VARNUM_MIN		LLONG_MIN
+#  define VARNUM_MAX		LLONG_MAX
+#  define UVARNUM_MAX		ULLONG_MAX
+# else
+#  define VARNUM_MIN		LONG_LONG_MIN
+#  define VARNUM_MAX		LONG_LONG_MAX
+#  define UVARNUM_MAX		ULONG_LONG_MAX
+# endif
 #endif
 
 typedef double	float_T;