(maint) Fix Clang issue with ignored GCC warning

GCC introduced a maybe-uninitialized warning that Clang doesn't
recognize, but Clang does try to interpret all GCC diagnostic pragmas.
Move the warning to be ignored in CMake; it's relatively new and likely
to cause problems with 3rd-party libraries.
Merge pull request #222 from MikaelSmith/maint/master/fix-clang-missing-warning

(maint) Fix Clang issue with ignored GCC warning