waf: accept env variable to append library

Change check_librt() to do everything it needs to check for librt.

Tools/ardupilotwaf/boards.py

@@ -184,12 +184,11 @@ class sitl(Board):
                 '-O3',
             ]
 
-        cfg.check_librt()
-
         env.LIB += [
             'm',
         ]
-        env.LIB += cfg.env.LIB_RT
+
+        cfg.check_librt(env)
 
         env.LINKFLAGS += ['-pthread',]
         env.AP_LIBRARIES += [
@@ -216,12 +215,11 @@ class linux(Board):
                 '-O3',
             ]
 
-        cfg.check_librt()
-
         env.LIB += [
             'm',
         ]
-        env.LIB += cfg.env.LIB_RT
+
+        cfg.check_librt(env)
 
         env.LINKFLAGS += ['-pthread',]
         env.AP_LIBRARIES = [

Tools/ardupilotwaf/cxx_checks.py

@@ -125,8 +125,8 @@ def ap_common_checks(cfg):
     )
 
 @conf
-def check_librt(cfg):
-    success = cfg.check(
+def check_librt(cfg, env):
+    ret = cfg.check(
         compiler='cxx',
         fragment='''
         #include <time.h>
@@ -140,10 +140,11 @@ def check_librt(cfg):
         mandatory=False,
     )
 
-    if success:
-        return success
+    if ret:
+        return ret
 
-    return cfg.check(
-        compiler='cxx',
-        lib='rt',
-    )
+    ret = cfg.check(compiler='cxx', lib='rt')
+    if ret:
+        env.LIB += cfg.env['LIB_RT']
+
+    return ret