Add a no-op setitimer() wrapper

The current code uses setitimer() only for reducing perceived
latency.  On platforms that lack setitimer() (e.g. HP NonStop),
allow builders to say "make NO_SETITIMER=YesPlease" to use a no-op
substitute, as doing so would not affect correctness.

HP NonStop does provide struct itimerval, but other platforms may
not, so this is taken care of in this commit too, by setting
NO_STRUCT_ITIMERVAL.

Signed-off-by: Joachim Schmitz <jojo@schmitz-digital.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -157,6 +157,11 @@ all::
 # Define NO_PREAD if you have a problem with pread() system call (e.g.
 # cygwin1.dll before v1.5.22).
 #
+# Define NO_SETITIMER if you don't have setitimer()
+#
+# Define NO_STRUCT_ITIMERVAL if you don't have struct itimerval
+# This also implies NO_SETITIMER
+#
 # Define NO_THREAD_SAFE_PREAD if your pread() implementation is not
 # thread-safe. (e.g. compat/pread.c or cygwin)
 #
@@ -1670,6 +1675,13 @@ endif
 ifdef OBJECT_CREATION_USES_RENAMES
 	COMPAT_CFLAGS += -DOBJECT_CREATION_MODE=1
 endif
+ifdef NO_STRUCT_ITIMERVAL
+	COMPAT_CFLAGS += -DNO_STRUCT_ITIMERVAL
+	NO_SETITIMER=YesPlease
+endif
+ifdef NO_SETITIMER
+	COMPAT_CFLAGS += -DNO_SETITIMER
+endif
 ifdef NO_PREAD
 	COMPAT_CFLAGS += -DNO_PREAD
 	COMPAT_OBJS += compat/pread.o

git-compat-util.h

@@ -162,6 +162,17 @@
 #define probe_utf8_pathname_composition(a,b)
 #endif
 
+#ifdef NO_STRUCT_ITIMERVAL
+struct itimerval {
+	struct timeval it_interval;
+	struct timeval it_value;
+}
+#endif
+
+#ifdef NO_SETITIMER
+#define setitimer(which,value,ovalue)
+#endif
+
 #ifndef NO_LIBGEN_H
 #include <libgen.h>
 #else