Fixes for i18n build on OS X:
* Adds ICU and tinygettext to build-osx-libs.sh.
* Updates ICU and tinygettext compiling/linking for OS X. icu-config is used to get ICU compiler flags and dependencies without hardcoding them in extern_libs4.lua.

Note: tinygettext is still missing some flags, so the OS X SDK build likely won't fully work yet.
Fixes tinygettext CMake script. Apparently it was only tested for glibc builds where iconv wasn't an external dependency, so it wasn't linked even for Windows / OS X. This is hopefully fixed by testing HAVE_ICONV_IN_LIBC which gets set by FindICONV.cmake (a related issue is how to detect this in our premake scripts).

Also disables two warnings which aren't supported by llvm-gcc 4.2.1 on OS X.