boost: Link Python when boost_python is fetched

[Doekin]

This PR attempts to address issues #6398 and #6399 by:
1. Preferring shared libraries for Boost when they are available on the system
2. linking Python when boost_python is present

[waruqi]

when package:config("shared") == false, use static libs, but shared libs are found. result will be shared libs.

[Doekin]

Thank you for your feedback. In #6399 , I encountered an issue where boost_test_exec_monitor and boost_exception were only available as static libraries, and were inadvertently mixed with shared libraries, causing a double free at runtime.

Typically, only shared libraries are installed, with static libraries sometimes included as well. Given that shared libraries take priority during linking, there's a risk of mistakenly linking shared libraries when the fetch result includes static ones.

Therefore, I guess preferring shared libraries might help avoid this issue.

[waruqi]

I know, but if package:config("shared") == false, we should only fetch static libraries.

if you want to use shared libraries, you should use add_requires("boost", {configs = {shared = true}})

[Doekin]

Sorry for the confusion, here’s my thought:

Currently, when static libraries are fetched, shared libraries are actually used due to their presence.

To clarify this situation and avoid mixing shared libraries (which are available both as shared and static) with pure static libraries (only available as static), it might make sense to always prefer shared libraries when possible, considering that add_requires("openssl") or add_deps("python") are common ways to fetch system-installed libraries, where package:config("shared") is false, yet shared libraries are still being fetched.

Alternatively, perhaps we could use add_linkgroups("a", "b", {static = true}) to ensure that static libraries are exactly used when fetched?

[waruqi]

If you want to ensure compatibility and stability, you should disable system libraries. add_requires("boost", {system = false})

Now many packages' system libraries cannot control whether to choose static libraries or dynamic libraries, and there are many uncertainties.

[Doekin]

OK, I'll explore other approaches to address my issue