waf: ignore -Wundef for gtest

The gtest header uses lots of undefined macros, showing lots of warnings
if we enable -Wundef. Ideally we could use a #pragma to ignore the
warning only from the correct header, but this currently doesn't work
with g++ - see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53431

So for now we disable the warning completely when compiling gtest or any
test that uses its header.

Thanks Gustavo Sousa

Tools/ardupilotwaf/ardupilotwaf.py

@@ -185,6 +185,7 @@ def ap_find_tests(bld, use=[]):
             program_name=f.change_ext('').name,
             program_group='tests',
             use_legacy_defines=False,
+            cxxflags=['-Wno-undef'],
         )
 
 @conf

Tools/ardupilotwaf/gtest.py

@@ -5,6 +5,7 @@
 gtest is a Waf tool for test builds in Ardupilot
 """
 
+from waflib import Utils
 from waflib.Configure import conf
 
 def configure(cfg):
@@ -25,6 +26,7 @@ def configure(cfg):
 
 @conf
 def libgtest(bld, **kw):
+    kw['cxxflags'] = Utils.to_list(kw.get('cxxflags', [])) + ['-Wno-undef']
     kw.update(
         source='modules/gtest/src/gtest-all.cc',
         target='gtest/gtest',