[bug-gperf] The gperf build script ignores $AR

[György Andrasek]

In order to build with clang's link-time optimization on Linux, the
toolchain needs to know about the LLVMgold plugin. For `ar`, this is
pretty much only possible with a script:

```
#!/bin/sh
firstarg=${1}
shift

exec /usr/bin/ar "${firstarg}" --plugin /usr/lib/LLVMgold.so "address@hidden"
```

Currently there is no way to tell gperf to build with this script, so
the build fails.