Cygwin: Enable build of SITL jMAVsim under Windows using the Cygwin Unix-like environment

Most of the incompatitbilities are luckily similar to the darwin build.
- New target OS __PX4_CYGWIN added because in other build environments on Windows defines will very likely be completely different
- added all necessary exeptions to the defines
- disabled priorities completely because on Windows they are defined 1-32 and with all the arbitrary +40 -40 priority settings there were a lot of problems
  not only did some threads/"virtual tasks" not start because of out of bound priorities but also the resulting scheduling was totally random and inadequate
  with default priorities it ran toally fine during my first tests, should be rethought when windows is used onboard in the future

platforms/posix/cmake/px4_impl_os.cmake

@@ -196,8 +196,17 @@ function(px4_os_add_flags)
 				)
 		endif()
 
-	else()
+	elseif(CYGWIN)
+		set(added_definitions
+			-D__PX4_POSIX
+			-D__PX4_CYGWIN
+			-D_GNU_SOURCE
+			-D__USE_LINUX_IOCTL_DEFS
+			-Dnoreturn_function=__attribute__\(\(noreturn\)\)
+			-U __CUSTOM_FILE_IO__
+			)
 
+	else()
 		set(added_definitions
 			-D__PX4_POSIX
 			-D__PX4_LINUX