Podspec describes process to build on Xcode 4.6.3

This is a modified version of the libkml.podspec that is included
in the Cocoapods specs repo.  It now uses subspecs to handle the
third-party libraries so they will be placed in the proper folders.
This in turn causes the #includes in the source files to resolve
properly without the need for special xcconfig switches.
Fixes build warnings in Xcode 4.6.3.

Format specifier mismatch and discarding of const qualifier.
Fixes compiler warnings with Xcode 4.6.3.

Mostly a bunch of if statements without braces.  Some extraneous
parens in conditionals.  Also the zlib functions are all prefixed
with libkml_ in this code tree, but the function calls were not
prefixed with libkml_.  This appears to have been a long-standing
configuration bug with the version of minizip used in this project.
I suspect that earlier builds were not actually using the embedded
version of minizip, but were using the version from the system
libraries. They are now using the functions with the libkml_ prefix.