Make __declspec(dllimport) optional (and off by default)

[roothorick]

The added comment sums up the rationale, but to elaborate... the two big problems avoided by omitting it:

• dllimport is not supported by GCC on non-Windows hosts, regardless of the target platform. In other words, when cross-compiling to Windows, dllimport is unavailable and will generate a compile-time error.
• The compiler takes this prefix as a hint that it can assume the method will be provided by a DLL, and therefore generates a direct call into the DLL. This doesn't fly if the library is actually being statically linked -- something the compiler can't know ahead of time.

Of course, there may be edge cases where not prefixing with dllimport will cause undesired behavior, or generate a more substantial relocation thunk with a significant performance impact. Those that find themselves dealing with such cases can simply define EPOXY_USE_DLLIMPORT where necessary.

This is a pretty good explanation of what __declspec(dllimport) actually does: http://blogs.msdn.com/b/russellk/archive/2005/03/20/399465.aspx?wa=wsignin1.0

[endrift]

I had to make a similar change to this when I made a static build for Windows. I'm not sure this approach is the best, but it's certainly better than the nothing this library has at the moment.

[yaronct]

@roothorick, @endrift: U may want to check my fork where I believe I've fixed this issue. It also supports building with CMake, including static builds, and several bug fixes.