Package glew: Enable shared build & use original makefile.

I made two different patches. I like this one better, it switches to using the upstream Makefile of glew.

There is another branch https://github.com/TobiX/mxe/tree/glew-shared, which only enables shared builds and doesn't change the build style (calls gcc and ld directly instead of using the upstream Makefile). Please select the one you like better or :8ball: ;)

Both use -O0 for shared builds, otherwise the resulting DLL does not work (don't know why) - see http://paste.debian.net/88601/ for a transcript with wine (For wine, this works with -O1, on Windows 7, it only works with -O0). I'll try to get behind this, but this seems to work for now...