docs: list all required build dependencies and clang version in BUILDING.md

[CEnnisgit]

Following BUILDING.md on a fresh Ubuntu 24.04 install currently fails at configure time:

• Could NOT find PkgConfig — 3rdparty/Vulkan-Loader hard-requires pkg-config (GitHub CI runners preinstall it, which is why CI never sees this)
• hidapi fails on the missing libudev pkg-config module → needs libudev-dev
• without the X11/Wayland dev headers, configure warns "Compiling Vulkan-Loader without X11 support" and GLFW loses its window-system backends

This PR updates the Debian dependency line to match what .github/workflows/rpcsx.yml already installs, and adds a short clang section documenting that clang ≥ 20 plus the matching clang-tools package (for clang-scan-deps) are required.

Verified on commit e8ae148, fresh Ubuntu 24.04 x86_64:

• g++-14: clean build after installing the listed packages
• clang-20 + clang-tools-20: clean build
• clang-18: fails on C++23 relaxed constexpr (P2448) in rx/StaticString.hpp and rpcsx/gpu/lib/gcn-shader/src/eval.cpp
• clang-19: fails on enum reflection in rx/format.hpp (-Wenum-constexpr-conversion not SFINAE-friendly until clang 20)

🤖 Generated with Claude Code

https://claude.ai/code/session_01WyB6A1ZejZwDLADJ9J525Y

[CEnnisgit]

Noting the overlap with #100, which flagged the missing libudev-dev first (credit to @kostakis) — this PR includes that fix and additionally covers pkg-config, the X11/Wayland headers, and the clang toolchain requirements.

It also answers the WSL question raised in #100's thread: on WSL2 Ubuntu 24.04, current master (e8ae148) configures and builds clean with both g++-14 and clang-20 + clang-tools-20 once the packages listed here are installed, and bin/rpcsx --help/-v run fine. The linker errors mentioned there (relocation truncated to fit, __cxa_call_terminate) did not reproduce with the stock Ubuntu 24.04 g++-14. Only caveat: WSL's Vulkan is llvmpipe (no dzn ICD in Ubuntu's mesa), so it's CPU rendering only there — build/dev works, GPU testing needs native Linux.