This change updates BUILD.md for Conan 2, add fixes/workarounds for Apple Clang 17, Clang 20 and CMake 4. This also removes (from BUILD.md only) workarounds for compiler versions which we no longer support e.g. Clang 15 and adds compilation flag -Wno-deprecated-declarations to enable building with Clang 20 on Linux.
On macOS, if you have not installed something that depends on `xz`, then your
system may lack `lzma`, resulting in a build error similar to:
```
Downloading libarchive-3.6.0.tar.xz completed [6250.61k]
libarchive/3.6.0:
ERROR: libarchive/3.6.0: Error in source() method, line 120
get(self, **self.conan_data["sources"][self.version], strip_root=True)
ReadError: file could not be opened successfully:
- method gz: ReadError('not a gzip file')
- method bz2: ReadError('not a bzip2 file')
- method xz: CompressionError('lzma module is not available')
- method tar: ReadError('invalid header')
```
The solution is to ensure that `lzma` is installed by installing `xz`.
* Remove obsolete build instructions.
* By using Conan, builders can choose which dependencies specifically to
build and link as shared objects.
* Refactor the build instructions based on the plan in #4433.
