Leverage std::string overloads in pugixml 1.15

[cboulay]

Close #247

This is a redo of #253 , originally by @myd7349. The only difference is that it is rebased on dev after I updated some GHA secrets. Either that, or the fact that this is coming from my account, should bypass the errors related to secret access.

Original text:

I have fixed the compilation error on Ubuntu.

Specifically, this error occurs when -DLSL_BUNDLED_PUGIXML=OFF is specified:

liblsl/.github/workflows/cppcmake.yml

Line 43 in 63741a9

- {name: "ubuntu-24.04-x64", os: "ubuntu-24.04", arch: "x86_64", cmake_extra: "-DLSL_BUNDLED_PUGIXML=OFF" }

pugixml 1.15 added std::string overloads (in fact, std::string_view) for some APIs, but older versions of pugixml do not support them. When liblsl is built against a non-bundled pugixml prior to 1.15, these issues arise. Therefore, to use the new overloads while remaining compatible with older pugixml versions, it is necessary to check whether the PUGIXML_HAS_STRING_VIEW macro is defined.

https://github.com/zeux/pugixml/blob/v1.15/src/pugixml.hpp#L445-L447

[cboulay]

I just wanted to verify that the apple tests pass, which they do. We can return to using #253