CLEANUP: remove THREAD_LOCAL from config.h

This one really ought to be defined in hathreads.h like all other thread
definitions, which is what this patch does. As expected, all files but
one (regex.h) were already including hathreads.h when using THREAD_LOCAL;
regex.h was fixed for this.

This was the last entry in config.h which is now useless.
diff --git a/include/common/hathreads.h b/include/common/hathreads.h
index cc54574..ae1009a 100644
--- a/include/common/hathreads.h
+++ b/include/common/hathreads.h
@@ -44,6 +44,7 @@
 
 #ifndef USE_THREAD
 
+#define THREAD_LOCAL  /* empty */
 #define MAX_THREADS 1
 #define MAX_THREADS_MASK 1
 
@@ -258,6 +259,8 @@
 #include <pthread.h>
 #include <import/plock.h>
 
+#define THREAD_LOCAL __thread
+
 #ifndef MAX_THREADS
 #define MAX_THREADS LONGBITS
 #endif