support for MinGW and cross compilation

[jpcima]

This patch modifies the cmake build system with two objectives: to support compilation under MinGW and cross-compilation from a different OS.

The current build uses the variables of the host system instead of the target. Instead, I rely on cmake's system name. It is the reliable method used by the internal modules of cmake.
I detect MinGW when the compiler identifies as either gcc or clang, and in that case, I choose to disable the standalone version because the environment lacks the necessary headers.

I have also included some necessary source code modifications. Lowercase includes are needed for a case sensitive filesystem. I have defined WIN32_LEAN_AND_MEAN to prevent conflicts of type names between expat and the winapi, and I have added 2 inclusions made necessary by this change.

[scheffle]

Hi,
thanks for this, but can you make two pull requests of this. One that handles case sensitivity includes and one with the cmake changes ? The cmake changes will take longer to verify.

[jpcima]

Thanks for merging my patches. To be frank about this one, I don't think it is interesting to divide the code as suggested, because I do it only for the sake of permitting windows cross-compilation; in the state of things it's never a problem since only windows is able to build the windows version.
I can do this later when I have finished with my task at hand.

[jpcima]

I have amended the branch to keep only the cmake changes, and I have edited the inclusions in the separate PR #38 as requested.