build: replace explicit gcc check by warning check

Check if the compiler has that warning instead of checking by the gcc
version. This allows clang to ignore those warnings as it understands
the same pragma.

Author: lucasdemarchi

configure.ac

@@ -140,6 +140,7 @@ AC_LANG_PUSH([C++])
 CC_CHECK_FLAGS_APPEND(with_cxxflags, [CXXFLAGS], [ \
 	-Wall \
 	-W \
+	-Waddress-of-packed-member \
 	-Wextra \
 	-Wno-inline \
 	-Wundef \
@@ -168,6 +169,9 @@ CC_CHECK_FLAGS_APPEND(with_cxxflags, [CXXFLAGS], [ \
 	-Wtype-limits \
 	-Wuninitialized])
 AC_LANG_POP([C++])
+
+AS_VAR_IF([cc_cv_CXXFLAGS__Waddress_of_packed_member], [yes],
+	  [with_cxxflags="$with_cxxflags -DHAVE_WADDRESS_OF_PACKED_MEMBER"])
 AC_SUBST([OUR_CXXFLAGS], "$with_cxxflags")
 
 #####################################################################

src/common/mavlink.h

@@ -13,7 +13,7 @@
  * mips, arm, etc that doesn't silently handle unaligned access,
  * you may be interested checking case by case in mavlink/pymavlink.
  */
-#if defined(__GNUC__) && __GNUC__ >= 9
+#if defined(HAVE_WADDRESS_OF_PACKED_MEMBER) && HAVE_WADDRESS_OF_PACKED_MEMBER
 #pragma GCC diagnostic ignored "-Waddress-of-packed-member"
 #endif
 #include <mavlink.h>