failure to build on OpenIndiana g++14

**Describe the bug**
Build failure.

And link failure if replacing with std::isnan.

**Expected behavior**
It building and linking properly.

**Reproduction steps**
Steps to reproduce the bug.
- install OpenIndiana/some illumos-based distro with this specific gcc version
- `sudo pkg install gcc-14`
- `git clone --depth=1 https://github.com/catchorg/Catch2`
- `cd Catch2`
- `cmake -DCMAKE_BUILD_TYPE=Release -B build`
- `cmake --build build -- -j128`

**Platform information:**

- OS: OpenIndiana
- Compiler+version: **GCC v14.3.0**
- Catch version: **latest git**

**Additional context**
I solved the issue by replacing with std::isnan, but it didn't make the linker happy either, Replacing Catch::isnan for std::isnan in catch_tostring.cpp makes it compile and link just fine.

Start with catch_tostring.cpp and reproduce the failure using the reported OpenIndiana environment, GCC 14.3.0, and the documented CMake build commands. Compare the Catch::isnan and std::isnan behavior, then verify that the project compiles and links successfully with the same build.

Written by the indexing model from the issue text.