Build broken out of the box on OS X

```
[ 81%] Linking CXX executable bin/collada2gltf
Undefined symbols for architecture x86_64:
"_libiconv", referenced from:
_xmlIconvWrapper in libxml2.a(encoding.o)
"_libiconv_close", referenced from:
_xmlFindCharEncodingHandler in libxml2.a(encoding.o)
_xmlCharEncCloseFunc in libxml2.a(encoding.o)
"_libiconv_open", referenced from:
_xmlFindCharEncodingHandler in libxml2.a(encoding.o)
"_lzma_auto_decoder", referenced from:
_xz_make in libxml2.a(xzlib.o)
"_lzma_code", referenced from:
_xz_decomp in libxml2.a(xzlib.o)
"_lzma_end", referenced from:
___libxml2_xzclose in libxml2.a(xzlib.o)
"_lzma_properties_decode", referenced from:
_xz_make in libxml2.a(xzlib.o)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[2]: *** [bin/collada2gltf] Error 1
make[1]: *** [CMakeFiles/collada2gltf.dir/all] Error 2
make: *** [all] Error 2
```

I suspect you need to call find_library for lzma and iconv in the CMake configuration, and add them to the target link libraries.

Start in the CMake configuration for the collada2gltf target and inspect how its target link libraries are defined. Reproduce the OS X build and verify that linking completes without the reported libiconv and lzma undefined symbols.

Written by the indexing model from the issue text.