Don't set $LD_RUN_PATH when empty

That lead to this failure when building in Fedora:

    ERROR   0010: file '/usr/lib64/python3.12/site-packages/mod_wsgi/server/mod_wsgi-py312.cpython-312-x86_64-linux-gnu.so' contains an empty runpath in []

Author: hroncok

setup.py

@@ -336,7 +336,8 @@ def get_apu_includes():
     LD_RUN_PATH += ':' + ':'.join(LD_RUN_PATHS)
     LD_RUN_PATH = LD_RUN_PATH.lstrip(':')
 
-    os.environ['LD_RUN_PATH'] = LD_RUN_PATH
+    if LD_RUN_PATH:
+        os.environ['LD_RUN_PATH'] = LD_RUN_PATH
 
 # On MacOS X, recent versions of Apple's Apache do not support compiling
 # Apache modules with a target older than 10.8. This is because it