fix multiple definitions of CDT variables under MinGW on Windows

fix multiple definitions of CDT variables under MinGW on Windows

Building with MinGW on Windows resulted in lots of build errors of the form:

  …/x86_64-w64-mingw32/bin/ld.exe: CMakeFiles\cdt.dir/objects.a(dtdisc.c.obj):
    dtdisc.c:(.bss+0x0): multiple definition of `Dtset';
    CMakeFiles\cdt.dir/objects.a(dtclose.c.obj):dtclose.c:(.bss+0x0): first
    defined here

The problem is that this environment defines _WIN32, so it sees the __declspec
alternatives for CDT_API in cdt.h. This is fine as GCC understands __declspec,
but it results in variable declarations in this header then missing the extern
qualifier. GCC’s semantics, in contrast to MSVC, interpret this as a definition
rather than a declaration.

The solution here is to *always* apply extern to these declarations, as MSVC is
happy with this as well. This is only addressing the immediate cause, and not
the longer term issue that __declspec(dllexport) and __declspec(dllimport) in a
Windows build should really be mapped to __attribute__((visibility("default")))
and __attribute__((visibility("hidden"))) in a non-Windows build, not to extern.

Thanks to @Synoecium and @swaldhoer for guidance on this fix.