Inject LDFLAGS into CXXFLAGS as fastjet-contrib doesn't accept them

This is the result of ./configure --help for fastjet-contrib:

```
Usage:
  configure [--help] [--list] [--fastjet-config=FILE] [--prefix=PREFIX] [CXX=...] [CXXFLAGS=...]
```

setup.py

@@ -106,8 +106,7 @@ def build_extensions(self):
                     "./configure",
                     f"--fastjet-config={FASTJET}/fastjet-config",
                     f'CXX={env["CXX"]}',
-                    "CXXFLAGS=-O3 -Bstatic -Bdynamic -std=c++17",
-                    f'LDFLAGS={env["LDFLAGS"]}',
+                    f'CXXFLAGS=-O3 -Bstatic -Bdynamic -std=c++17 {env["LDFLAGS"]}',
                 ],
                 cwd=FASTJET_CONTRIB,
                 env=env,