waf: create taskgen for gtest in _build_common_taskgens

So that it belongs to 'build' group.

Tools/ardupilotwaf/gtest.py

@@ -5,6 +5,8 @@
 gtest is a Waf tool for test builds in Ardupilot
 """
 
+from waflib.Configure import conf
+
 def configure(cfg):
     cfg.env.HAS_GTEST = False
 
@@ -27,11 +29,13 @@ def configure(cfg):
 
     cfg.env.HAS_GTEST = True
 
-def build(bld):
-    bld.stlib(
+@conf
+def libgtest(bld, **kw):
+    kw.update(
         source='modules/gtest/src/gtest-all.cc',
         target='gtest/gtest',
         includes='modules/gtest/ modules/gtest/include',
         export_includes='modules/gtest/include',
         name='GTEST',
     )
+    return bld.stlib(**kw)

wscript

@@ -155,6 +155,8 @@ def _build_common_taskgens(bld):
         use='mavlink',
     )
 
+    bld.libgtest()
+
 def _build_recursion(bld):
     common_dirs_patterns = [
         # TODO: Currently each vehicle also generate its own copy of the
@@ -203,7 +205,6 @@ def build(bld):
     bld.post_mode = Build.POST_LAZY
 
     bld.load('ardupilotwaf')
-    bld.load('gtest')
 
     _build_cmd_tweaks(bld)