osdep.h: Make TIME_MAX handle different time_t types

In our various supported host OSes, the time_t type may be either 32
or 64 bit, and could in theory also be either signed or unsigned.
Notably, in OpenBSD time_t is a 64 bit type even if 'long' is 32
bits, so using LONG_MAX for TIME_MAX is incorrect.

Use an approach suggested by Paolo Bonzini which calculates
the maximum value of the type rather than hardcoding it;
to do this we use the TYPE_MAXIMUM macro from Gnulib.

Backports commit e7b47c22e2df14d55e3e4426688c929bf8e3f7fb from qemu

qemu/include/qemu/osdep.h

@@ -127,8 +127,34 @@
 #if !defined(EMEDIUMTYPE)
 #define EMEDIUMTYPE 4098
 #endif
+
+/* time_t may be either 32 or 64 bits depending on the host OS, and
+ * can be either signed or unsigned, so we can't just hardcode a
+ * specific maximum value. This is not a C preprocessor constant,
+ * so you can't use TIME_MAX in an #ifdef, but for our purposes
+ * this isn't a problem.
+ */
+
+/* The macros TYPE_SIGNED, TYPE_WIDTH, and TYPE_MAXIMUM are from
+ * Gnulib, and are under the LGPL v2.1 or (at your option) any
+ * later version.
+ */
+
+/* True if the real type T is signed.  */
+#define TYPE_SIGNED(t) (!((t)0 < (t)-1))
+
+/* The width in bits of the integer type or expression T.
+ * Padding bits are not supported.
+ */
+#define TYPE_WIDTH(t) (sizeof(t) * CHAR_BIT)
+
+/* The maximum and minimum values for the integer type T.  */
+#define TYPE_MAXIMUM(t)                                                \
+  ((t) (!TYPE_SIGNED(t)                                                \
+        ? (t)-1                                                        \
+        : ((((t)1 << (TYPE_WIDTH(t) - 2)) - 1) * 2 + 1)))
 #ifndef TIME_MAX
-#define TIME_MAX LONG_MAX
+#define TIME_MAX TYPE_MAXIMUM(time_t)
 #endif
 
 /* HOST_LONG_BITS is the size of a native pointer in bits. */