waf: only enable -Wsizeof-pointer-div for gcc v8.4 or greater

The -Wsizeof-pointer-div option doesn't exist in gcc v7.5, which is the
version used by Ubuntu 18.04 LTS. It does exist in v8.4.

Tools/ardupilotwaf/boards.py

@@ -319,6 +319,9 @@ class Board:
                     '-Werror=implicit-fallthrough',
                     '-Werror=maybe-uninitialized',
                     '-Werror=duplicated-cond',
+                ]
+            if self.cc_version_gte(cfg, 8, 4):
+                env.CXXFLAGS += [
                     '-Werror=sizeof-pointer-div',
                 ]