Simplify DLL creation

[jdm]

Based on https://stackoverflow.com/questions/2384932/converting-static-link-library-to-dynamic-dll, we should be able to avoid building all of the source files twice when building a static library as well as a DLL. Given the overlap between the source files for libANGLE and the other static libraries, it seems like it should be possible to filter out any duplicates and just link to libANGLE while building instead, as well.