Fix shared library build on OS X

[dean0x7d]

I noticed this while trying to run format-benchmark. Building cppformat on OS X with BUILD_SHARED_LIBS=ON results in:

ld: unknown option: --as-needed
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[2]: *** [libcppformat.1.2.0.dylib] Error 1
make[1]: *** [CMakeFiles/cppformat.dir/all] Error 2
make: *** [all] Error 2

This PR fixes it.

As a side note: The shared lib build is also broken with MSVC and it's a bit more elaborate. I don't think this is a big issue since a shared lib is probably not the optimal way to use cppformat. But just FYI.

This commit will resolve the ignoring unknown flag --as-needed warning on MSVC, but the build will still fail because it needs __declspec(dllexport) and __declspec(dllimport) to know which functions and classes to export. Those need to be behind macro defines and then peppered all over the code which can get quite ugly. Template instantiations can also complicate matters. The code uglification is probably a high price to pay for an almost-never-used feature. I am not advocating for it, but I can take a stab at making it work if there is interest.

[vitaut]

Thanks for the PR. Wouldn't it be better to check for CMAKE_COMPILER_IS_GNUCXX instead because the issue is probably specific to clang and not OS X?

Regarding the DLL API, I'd rather not introduce it at this point unless there are strong reasons to do so because it will clutter the code.

[dean0x7d]

It's a linker flag so it's platform specific. GCC 5.2 (the real one, not the clang alias) on OS X still calls the same ld and fails:

ld: unknown option: --as-needed
collect2: error: ld returned 1 exit status
make[2]: *** [libcppformat.1.2.0.dylib] Error 1
make[1]: *** [CMakeFiles/cppformat.dir/all] Error 2
make: *** [all] Error 2

[vitaut]

Good point. Too bad CMake doesn't seem to have a method to check for linker flags.