[libsigcplusplus] Add 2.12.1, + some formatting changes

[EZ4Stephen]

-Add support for libsigcplusplus 2.12.1.

-Some formatting changes to libsigcplusplus' xmake.lua:
--Add an on_load block to handle include path for 2.x.x and 3.x.x versions differently.
--Change the add_url link, and the sha256 for all versions of libsigcplusplus

After first adding the 2.12.1 version and sha256, I got an error relating to an untracked folder from meson, which subsequently made xmake errors:
sigc++\meson.build:155:19: ERROR: File ../untracked/sigc++/adaptors/lambda/lambda.cc does not exist.

Looking into the cause, found some insightful sources:

1. 2.10.3: meson broken libsigcplusplus/libsigcplusplus#62
2. https://github.com/libsigcplusplus/libsigcplusplus#building-from-a-release-tarball

At least for the versions in the build files, doesn't seem like there's a need to define maintainer-mode.
Tested all 4 versions locally (Windows). Don't know if my changes adds support for any more platforms.

This closes #9852 .

[gemini-code-assist]

Code Review

This pull request updates the libsigcplusplus package to use release download URLs, updates version hashes, and adds version 2.12.1. It also introduces a dynamic on_load block to set include directories based on the package's major version. A potential issue was identified where non-semver versions could cause a Lua error during string concatenation, and a more robust implementation was suggested to handle these cases.