Fix CMake to support being a subproject

[wgtmac]

We borrowed a lot of CMake implementation from Apache Arrow (including function names and module file names). The conflicting names make it difficult to be used as a subproject. This PR fix this by adding a Iceberg prefix.

Closes #28

[wgtmac]

@zhjwpku Could you please try this?

cmake_minimum_required(VERSION 3.25)

project(dummy
        VERSION 0.1.0
        LANGUAGES CXX)

include(FetchContent)
fetchcontent_declare(Iceberg
                     GIT_REPOSITORY https://github.com/wgtmac/iceberg-cpp.git
                     GIT_TAG 19ea3f03895205a658aca083b66b732d527398a2)
set(ICEBERG_ARROW ON CACHE BOOL "" FORCE)
fetchcontent_makeavailable(Iceberg)

message(STATUS "iceberg_SOURCE_DIR: ${iceberg_SOURCE_DIR}")
message(STATUS "iceberg_BINARY_DIR: ${iceberg_BINARY_DIR}")

[zhjwpku]

It works! Thanks for the quick fix ;)

[wgtmac]

Thanks for the confirmation! @zhjwpku

@Fokko @Xuanwo Could you please merge this? It is a minor fix.

[Xuanwo]

Thank you @wgtmac for the quick fix. Let's move.