Build fails if build directory name includes a comma
Description of problem:
Builds fail if the build directory name contains a comma.
Steps to reproduce:
1. `mkdir build,demo && cd build,demo`
2. `../configure && make`

The linker fails because it uses a wrong build path (comma and trailing part of directory name is missing):

```
ld: can't read -exported_symbols_list file: /Users/stefan/src/gitlab/qemu-project/qemu/build
clang: error: linker command failed with exit code 1 (use -v to see invocation)
ninja: build stopped: subcommand failed.
```