Fix symbol visibility on Linux when compiling with -fvisibility=hidden

[milianw]

Make FMT_API symbols use the default visibility on non-Windows
platforms. Otherwise, one cannot use the generated fmt library when
compiling globally with -fvisibility=hidden.

Fixes compile errors like:

../3rdParty/fmt/include/fmt/core.h:757: error: undefined reference to 'fmt::v6::internal::assert_fail(char const*, int, char const*)'

Note that the symbol exists, but is local:

$ nm -C libfmtd.so.6.1.3  | grep assert_fail
                 U __assert_fail
0000000000233ffa t fmt::v6::internal::assert_fail(char const*, int, char const*)

With this patch, the compile error is gone and the symbol is properly
exported:

$ nm -a bin/libfmtd.so -C | grep assert_fail
                 U __assert_fail
00000000002366ba T fmt::v6::internal::assert_fail(char const*, int, char const*)

I agree that my contributions are licensed under the {fmt} license, and agree to future changes to the licensing.

[vitaut]

Thanks for the PR. Looks good but please fix the CI errors.

[milianw]

I'm quite puzzled by the CI errors, since I cannot reproduce them locally. Apparently it's testing some different compile configuration there than what I have locally. I'll have to find the time to figure out what's going on here. If you have any hints that would be appreciated.

[milianw]

Ah, -Werror explains it - I do see it locally after all...

[milianw]

this should fix the linux build, but I wonder if the FMT_API in the .cc file was required for Windows?

[vitaut]

I wonder if the FMT_API in the .cc file was required for Windows?

AFAIK yes, it is required by MSVC. I suggest introducing another macro (sigh) FMT_INSTANTIATION_DEF and using it on the instantiation definition instead of FMT_API.

[milianw]

With the AppVeyor build passing... do we really need this? Shouldn't that test this API too, just like the Linux CI on Travis did?

[vitaut]

do we really need this?

We do. CI doesn't test all possible configurations, just several common ones.

[vitaut]

Thanks!