make minja packagable by setting target in cmake

[jhlee525]

Functionality of the minja library works well, and I think it's a great library. But it lacks proper CMake packaging, which makes it difficult to integrate smoothly within other CMake-based projects.

This PR introduces the following changes:

1. Install Command
   The install command has been added so that running running make install will let you put the header files into $CMAKE_INSTALL_PREFIX/include/minja.

2. FetchContent Integration
   A CMake target has been defined to support users who want to include minja using FetchContent. This means other C++ projects to can import minja like this:

FetchContent_Declare(minja URL "minja-url")
FetchContent_MakeAvailable(minja)

3. Optional Python Dependency
   Currently, minja's CMake configuration requires a Python interpreter. For users who only need the header files, two options have been added: MINJA_TEST_ENABLED and MINJA_EXAMPLE_ENABLED. By disabling these options, users can skip unnecessary CMake steps and avoid the Python dependency when it is not needed.

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[jhlee525]

CI fails in reading files, for example

3: C++ exception with description "Failed to open file: tests/CohereForAI-c4ai-command-r7b-12-2024-tool_use.jinja" thrown in the test body.

However, I have no idea how test can get these files in CI

[ochafik]

Can confirm your branch runs great locally 👌. CI needs a HuggingFace key and I haven't sorted out a setup that prevents leaking it yet, sorry about this.

Looks like cmake --install build installs gtest & gmock too, wonder if there's a way to prevent this (besides disabling tests)? And maybe add a section to the top-level README.md w/ similar instructions to this PR's description?

[jhlee525]

I missed testing install with test option. It turns out that googletest having INSTALL_GTEST in CMakeLists.txt which forces installing itself (when using FetchContents).

So I put a line to disable this option.

[jhlee525]

Also, as well as instruction

[ochafik]

Thanks a lot @jhlee525, merged!!