maintainer-feedback requested: [Bug 260706] www/firefox: build fails with DEFAULT_VERSIONS+= llvm=13

Bugzilla Automation <bugzilla@FreeBSD.org> has asked freebsd-gecko (Nobody)
<gecko@FreeBSD.org> for maintainer-feedback:
Bug 260706: www/firefox: build fails with DEFAULT_VERSIONS+= llvm=13
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=260706



--- Description ---
From poudriere build log:

```
===>   firefox-95.0.2,2 depends on file:
/usr/local/llvm13/lib/clang//lib/wasi/libclang_rt.builtins-wasm32.a - not found
===>   Installing existing package /packages/All/wasi-compiler-rt13-13.0.0.pkg
[13amd64-usrports] Installing wasi-compiler-rt13-13.0.0...
[13amd64-usrports] Extracting wasi-compiler-rt13-13.0.0: .... done
===>   firefox-95.0.2,2 depends on file:
/usr/local/llvm13/lib/clang//lib/wasi/libclang_rt.builtins-wasm32.a - not found
```

This occurs because of Mk/bsd.gecko.mk:L96#L99

```
  95 # Ignore Mk/bsd.default-versions.mk but respect make.conf(5) unless LTO is
enabled
  96 .if !defined(DEFAULT_VERSIONS) || ! ${DEFAULT_VERSIONS:Mllvm*} ||
${PORT_OPTIONS:MLTO}
  97 LLVM_DEFAULT=   13 # chase bundled LLVM in lang/rust for LTO
  98 LLVM_VERSION=   13.0.0 # keep in sync with
devel/wasi-compiler-rt${LLVM_DEFAULT}
  99 .endif
```

So with no LTO but llvm=13 explicitly set in make.conf DEFAULT_VERSIONS, it
fails to set LLVM_VERSION and thus can't find the necessary file.

Possibly fix by checking for ${DEFAULT_VERSIONS:Mllvm13}