CMake: Use a better FindIconv.cmake

[rcdailey]

The current FindIconv.cmake module matches a library named "c", which happens to match 'libc.so', which is invalid. I replaced it with a version of FindIconv.cmake from the Orphis/boost-cmake project on github here:

https://github.com/Orphis/boost-cmake/blob/master/cmake/Modules/FindIconv.cmake

Tested in the following configurations:

• Cross compiling on Windows to Android via x86 & ARM architecture
• Compiling natively on Windows using MSVC 15

[aidansteele]

Thanks, this is great!