posix add -Wno-varargs for OS X

cmake/common/px4_base.cmake

@@ -490,6 +490,7 @@ function(px4_add_common_flags)
 		-Wpointer-arith
 		-Wmissing-declarations
 		-Wno-unused-parameter
+		-Wno-varargs
 		-Werror=format-security
 		-Werror=array-bounds
 		-Wfatal-errors

cmake/toolchains/Toolchain-posix-clang-native.cmake

@@ -26,6 +26,7 @@ set(WARNINGS
     -Werror=unused-private-field
     -Wno-packed
     -Wno-frame-larger-than=
+    -Wno-varargs
     #-Wcast-qual  - generates spurious noreturn attribute warnings,
     #               try again later
     #-Wconversion - would be nice, but too many "risky-but-safe"
@@ -59,6 +60,7 @@ set(C_FLAGS
 #
 set(CXX_WARNINGS
 	-Wno-missing-field-initializers
+	-Wno-varargs
 	)
 set(CXX_FLAGS
 	-fno-exceptions