waf: gtest: disable for PX4 boards

Google Test code uses functions signatures not implemented by the current
toolchain for PX4.

Tools/ardupilotwaf/gtest.py

@@ -8,7 +8,19 @@ gtest is a Waf tool for test builds in Ardupilot
 from waflib import Utils
 from waflib.Configure import conf
 
+import boards
+
 def configure(cfg):
+    board = boards.get_board(cfg.env.BOARD)
+    if isinstance(board, boards.px4):
+        # toolchain is currently broken for gtest
+        cfg.msg(
+            'Gtest',
+            'PX4 boards currently don\'t support compiling gtest',
+            color='YELLOW',
+        )
+        return
+
     cfg.env.HAS_GTEST = False
 
     if cfg.env.STATIC_LINKING: