BLD: Don't specify runtime_library_dirs on Cygwin

Closes pyproj4#1113 (pyproj4#1113 (comment)), pypa/distutils#171

runtime_library_dirs isn't supported by PE/COFF, used by Windows and Cygwin; it seems to be an ELF thing.

setup.py

@@ -167,7 +167,7 @@ def get_extension_modules():
  ext_options = {
  "include_dirs": include_dirs,
  "library_dirs": library_dirs,
- "runtime_library_dirs": library_dirs if os.name != "nt" else None,
+ "runtime_library_dirs": library_dirs if os.name != "nt" and sys.platform != "cygwin" else None,
  "libraries": get_libraries(library_dirs),
  }
  # setup cythonized modules