Fix some conditional compilation in kernel build mode. And, while we are...

Fix some conditional compilation in kernel build mode.  And, while we are touching this logic, make g_default_pthread_attr const.

include/nuttx/pthread.h

@@ -70,11 +70,11 @@ extern "C" {
 #define EXTERN extern
 #endif
 
-/* Default pthread attributes (see sched/pthread_create.c).  This global
- * can only be shared within the kernel- or within the user- address space.
+/* Default pthread attributes.  This global can only be shared within the
+ * kernel- or within the user- address space.
  */
 
-EXTERN pthread_attr_t g_default_pthread_attr;
+EXTERN const pthread_attr_t g_default_pthread_attr;
 
 /****************************************************************************
  * Public Function Prototypes

libc/pthread/pthread_attrinit.c

@@ -58,15 +58,15 @@
  * Global Variables
  ****************************************************************************/
 
-/* Default pthread attributes (see included/nuttx/pthread.h).  When configured
+/* Default pthread attributes (see include/nuttx/pthread.h).  When configured
  * to build separate kernel- and user-address spaces, this global is
  * duplicated in each address spaced.  This copy can only be shared within
  * the user address space.
  */
 
-#if (defined(CONFIG_BUILD_PROTECTED) && !defined(__KERNEL__)) && \
-    !defined(CONFIG_BUILD_KERNEL)
-pthread_attr_t g_default_pthread_attr = PTHREAD_ATTR_INITIALIZER;
+#if (defined(CONFIG_BUILD_PROTECTED) || defined(CONFIG_BUILD_KERNEL)) && \
+    !defined(__KERNEL__)
+const pthread_attr_t g_default_pthread_attr = PTHREAD_ATTR_INITIALIZER;
 #endif
 
 /****************************************************************************

sched/pthread/pthread_create.c

@@ -68,10 +68,13 @@
 /****************************************************************************
  * Global Variables
  ****************************************************************************/
+/* Default pthread attributes (see include/nuttx/pthread.h).  When configured
+ * to build separate kernel- and user-address spaces, this global is
+ * duplicated in each address spaced.  This copy can only be shared within
+ * the kernel address space.
+ */
 
-/* Default pthread attributes */
-
-pthread_attr_t g_default_pthread_attr = PTHREAD_ATTR_INITIALIZER;
+const pthread_attr_t g_default_pthread_attr = PTHREAD_ATTR_INITIALIZER;
 
 /****************************************************************************
  * Private Variables
@@ -242,7 +245,7 @@ int pthread_create(FAR pthread_t *thread, FAR pthread_attr_t *attr,
 
   if (!attr)
     {
-      attr = &g_default_pthread_attr;
+      attr = (FAR pthread_attr_t *)&g_default_pthread_attr;
     }
 
   /* Allocate a TCB for the new task. */