Emscripten build failing under Rust 1.93.0

Due to https://github.com/rust-lang/rust/issues/112195 we (or cc-rs [in the future maybe](https://github.com/rust-lang/rust/issues/112195#issuecomment-3476858918)) need to pass `-fwasm-exceptions` when building mupdf under Rust version 1.93.0 or later (or maybe depending on some env var, I'm unsure). mupdf's Makefile already does this since https://github.com/ArtifexSoftware/mupdf/commit/ddd718c916261c8e8464efab19d6ca115c8ec766 when passing `OS=wasm` (which we consciously aren't) but it confirms that they have support for this (no indirect calls to `setjmp`).