Fix an another -Wundef

[eliaskosunen]

Here's yet another -Wundef PR. Today we have FMT_CLANG_VERSION being used without being defined if FMT_GCC_VERSION is less than 600.

Even though the usage of an undefined macro has the well-defined semantics of evaluating to zero, I think doing so is bad practice and ignoring those warnings can easily lead to unintended behavior. I think that a project as widely used as fmt should compile cleanly with the most aggressive warning levels. To accomplish this, I really think CI should be more strict with warnings and use more compilers than just g++-6. I can work on a PR on that as well if you'd like.

[eliaskosunen]

For example, currently when compiling with clang you'll receive a boatload of warnings, including -Wsign-conversion, because these are only disabled for g++.

I've had to resort to doing this before including fmt:

#if defined(__GNUC__) || defined(__clang__)
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wswitch-enum"
#pragma GCC diagnostic ignored "-Wsign-conversion"
#pragma GCC diagnostic ignored "-Wconversion"
#pragma GCC diagnostic ignored "-Wformat-nonliteral"
#pragma GCC diagnostic ignored "-Wmissing-noreturn"
#pragma GCC diagnostic ignored "-Wunused-parameter"
#pragma GCC diagnostic ignored "-Wdeprecated"
#pragma GCC diagnostic ignored "-Wshadow"
#pragma GCC diagnostic ignored "-Wundef"

#if (defined(__GNUC__) && !defined(__clang__)) || \
    (defined(__clang__) && __clang_major__ > 4)
#pragma GCC diagnostic ignored "-Wzero-as-null-pointer-constant"
#endif
#if defined(__clang__) && __clang_major__ >= 4
#pragma GCC diagnostic ignored "-Wdeprecated-dynamic-exception-spec"
#endif

#ifdef __clang__
#pragma GCC diagnostic ignored "-Wshadow-field-in-constructor"
#pragma GCC diagnostic ignored "-Wdocumentation-unknown-command"
#pragma GCC diagnostic ignored "-Wunused-member-function"
#pragma GCC diagnostic ignored "-Wshorten-64-to-32"
#pragma GCC diagnostic ignored "-Wextra-semi"
#else
#pragma GCC diagnostic ignored "-Wctor-dtor-privacy"
#endif
#endif  // defined(__GNUC__) || defined(__clang__)

I think these should be either fixed or explicitly disabled in the header.

[vitaut]

I really think CI should be more strict with warnings and use more compilers than just g++-6. I can work on a PR on that as well if you'd like.

I think it would be useful to add an old gcc compiler (4.4.x) to the CI config. PRs for this and stricter warnings are welcome.

[vitaut]

Merged, thanks!