Fails to build with gcc 5.4.0 in C++14 mode

[reddwarf69]

When trying to build spdlog 1.12.0 with gcc 5.4.0 in C++14, we get

bash-5.2# cmake -B build -DSPDLOG_BUILD_BENCH=OFF -DSPDLOG_BUILD_EXAMPLE=OFF -DSPDLOG_BUILD_TESTS=OFF -DCMAKE_CXX_STANDARD=14 -DCMAKE_CXX_COMPILER=<path_to_gcc_54>
-- The CXX compiler identification is GNU 5.4.0
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: <path_to_gcc_54> - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Build spdlog: 1.12.0
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD - Failed
-- Looking for pthread_create in pthreads
-- Looking for pthread_create in pthreads - not found
-- Looking for pthread_create in pthread
-- Looking for pthread_create in pthread - found
-- Found Threads: TRUE  
-- Build type: Release
-- Generating install
-- Configuring done (0.6s)
-- Generating done (0.0s)
-- Build files have been written to: /spdlog-1.12.0/build
bash-5.2# cmake --build build 
[ 12%] Building CXX object CMakeFiles/spdlog.dir/src/spdlog.cpp.o
In file included from /spdlog-1.12.0/include/spdlog/spdlog.h:12:0,
                 from /spdlog-1.12.0/include/spdlog/spdlog-inl.h:7,
                 from /spdlog-1.12.0/src/spdlog.cpp:8:
/spdlog-1.12.0/include/spdlog/common.h: In function 'constexpr spdlog::string_view_t spdlog::details::to_string_view(const memory_buf_t&)':
/spdlog-1.12.0/include/spdlog/common.h:351:42: error: call to non-constexpr function 'const T* fmt::v9::detail::buffer<T>::data() const [with T = char]'
     return spdlog::string_view_t{buf.data(), buf.size()};
                                          ^
gmake[2]: *** [CMakeFiles/spdlog.dir/build.make:76: CMakeFiles/spdlog.dir/src/spdlog.cpp.o] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:83: CMakeFiles/spdlog.dir/all] Error 2
gmake: *** [Makefile:156: all] Error 2
bash-5.2#

The problem being that fmt only defines FMT_CONSTEXPR in gcc 6+ (https://github.com/gabime/spdlog/blob/v1.12.0/include/spdlog/fmt/bundled/core.h#L95) while spdlog just does a C++14 support check (https://github.com/gabime/spdlog/blob/v1.12.0/include/spdlog/common.h#L71).

Given https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67371, and spdlog using fmt functions supposedly affected by it, I guess spdlog should do the same check?

[tt4g]

If you can fix that, you should send a PR.