thread-utils: macros to unconditionally compile pthreads API

When built with NO_PTHREADS, the macros are used make the code build
even though pthreads header and library may be missing. The code can
still have different code paths for no threads support with
HAVE_THREADS variable.

There are of course impacts on no-pthreads builds:

- data structure may get slightly bigger because all the mutexes and
  pthread_t are present (as an int)

- code execution is not impacted much. Locking (in hot path) is
  no-op. Other wrapper function calls really should not matter much.

- the binary size grows bigger because of threaded code. But at least
  on Linux this does not matter, if some code is not executed, it's
  not mapped in memory.

This is a preparation step to remove "#ifdef NO_PTHREADS" in the code
mostly because of maintainability. As Jeff put it

> it's probably OK to stop thinking of it as "non-threaded platforms
> are the default and must pay zero cost" and more as "threaded
> platforms are the default, and non-threaded ones are OK to pay a
> small cost as long as they still work".

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

thread-utils.c

@@ -20,6 +20,9 @@
 
 int online_cpus(void)
 {
+#ifdef NO_PTHREADS
+	return 1;
+#else
 #ifdef _SC_NPROCESSORS_ONLN
 	long ncpus;
 #endif
@@ -59,10 +62,12 @@ int online_cpus(void)
 #endif
 
 	return 1;
+#endif
 }
 
 int init_recursive_mutex(pthread_mutex_t *m)
 {
+#ifndef NO_PTHREADS
 	pthread_mutexattr_t a;
 	int ret;
 
@@ -74,4 +79,47 @@ int init_recursive_mutex(pthread_mutex_t *m)
 		pthread_mutexattr_destroy(&a);
 	}
 	return ret;
+#else
+	return 0;
+#endif
 }
+
+#ifdef NO_PTHREADS
+int dummy_pthread_create(pthread_t *pthread, const void *attr,
+			 void *(*fn)(void *), void *data)
+{
+	/*
+	 * Do nothing.
+	 *
+	 * The main purpose of this function is to break compiler's
+	 * flow analysis and avoid -Wunused-variable false warnings.
+	 */
+	return ENOSYS;
+}
+
+int dummy_pthread_init(void *data)
+{
+	/*
+	 * Do nothing.
+	 *
+	 * The main purpose of this function is to break compiler's
+	 * flow analysis or it may realize that functions like
+	 * pthread_mutex_init() is no-op, which means the (static)
+	 * variable is not used/initialized at all and trigger
+	 * -Wunused-variable
+	 */
+	return ENOSYS;
+}
+
+int dummy_pthread_join(pthread_t pthread, void **retval)
+{
+	/*
+	 * Do nothing.
+	 *
+	 * The main purpose of this function is to break compiler's
+	 * flow analysis and avoid -Wunused-variable false warnings.
+	 */
+	return ENOSYS;
+}
+
+#endif