[pthreads] Add PTHREAD_NUM_MAX in Kconfig.

components/libc/Kconfig

@@ -8,6 +8,12 @@ config RT_USING_PTHREADS
     bool "Enable pthreads APIs"
     default n
 
+if RT_USING_PTHREADS
+    config PTHREAD_NUM_MAX
+    int "Maximum number of pthreads"
+    default 8
+endif
+
 if RT_USING_LIBC && RT_USING_DFS
     config RT_USING_POSIX
         bool "Enable POSIX layer for poll/select, stdin etc"

components/libc/pthreads/pthread_internal.h

@@ -30,7 +30,10 @@ struct _pthread_key_data
 };
 typedef struct _pthread_key_data _pthread_key_data_t;
 
+#ifndef PTHREAD_NUM_MAX
 #define PTHREAD_NUM_MAX 32
+#endif
+
 #define PTHREAD_MAGIC   0x70746873
 struct _pthread_data
 {