build: shut up cmake regarding deprecated macros

Yes, we know we are using them. Remove warnings like:

CMake Deprecation Warning at /usr/share/cmake/Modules/CMakeForceCompiler.cmake:93 (message):
  The CMAKE_FORCE_CXX_COMPILER macro is deprecated.  Instead just set
  CMAKE_CXX_COMPILER and allow CMake to identify the compiler.
Call Stack (most recent call first):
  cmake/toolchains/Toolchain-arm-none-eabi.cmake:37 (cmake_force_cxx_compiler)
  /home/lucas/p/dronecode/ardupilot/build/px4-v2/modules/PX4Firmware/CMakeFiles/3.6.2/CMakeSystem.cmake:6 (include)
  CMakeLists.txt:204 (project)

Tools/ardupilotwaf/px4.py

@@ -286,6 +286,7 @@ def build(bld):
         name='px4',
         cmake_src=bld.srcnode.find_dir('modules/PX4Firmware'),
         cmake_vars=bld.env.PX4_CMAKE_VARS,
+        cmake_flags=['-Wno-deprecated'],
     )
 
     px4.build(