aws-sdk-cpp: Fix library and include paths in generated cmake files

AWS's SDK by default tries to prepend its install root to each of the
library paths; this obviously fails with the absolute paths that Nix
gives it. Worse, it computes the installation root by walking up the
filesystem from its cmake file, so even if the AWSSDK_ROOT_DIR is
explicitly set to the root directory, it gets replaced with the path
to the derivation's dev output.

This is all fixed with a patch to the cmake files that generate the
installed configuration.

Once this is fixed, it *still* doesn't work because the export
generator built into cmake insists on adding `$out/include` to the
header search path; when importing this configuration in another
package, cmake will fail because `$out/include` doesn't exist (After
all, it was relocated by a fixup hook). A small postFixupHook will
recreate the directory and make cmake happy.