waf : use LINKFLAGS for pthread

-pthread is the one supposed to be used because it defines pre-defined
macros as -lpthread doesn't, like was done in

Tools/ardupilotwaf/boards.py

@@ -91,9 +91,8 @@ def sitl(env):
 
     env.LIB += [
         'm',
-        'pthread',
     ]
-
+    env.LINKFLAGS += ['-pthread',]
     env.AP_LIBRARIES += [
         'AP_HAL_SITL',
         'SITL',
@@ -114,10 +113,9 @@ def linux(env):
 
     env.LIB += [
         'm',
-        'pthread',
         'rt',
     ]
-
+    env.LINKFLAGS += ['-pthread',]
     env.AP_LIBRARIES = [
         'AP_HAL_Linux',
     ]

Tools/ardupilotwaf/gbenchmark.py

@@ -91,7 +91,7 @@ def configure(cfg):
 
     env.INCLUDES_GBENCHMARK = [prefix_node.make_node('include').abspath()]
     env.LIBPATH_GBENCHMARK = [prefix_node.make_node('lib').abspath()]
-    env.LIB_GBENCHMARK = ['benchmark','pthread']
+    env.LIB_GBENCHMARK = ['benchmark']
 
     env.HAS_GBENCHMARK = True