[benmoss]

In 2.0, _extension_flags() no longer reads PYMUPDF_MUPDF_LIB, although the docstring at the top of setup.py still documents it and 1.28.0 used it. When building against system MuPDF (PYMUPDF_SETUP_MUPDF_BUILD=), libpaths is therefore None.

On Linux the build still works if LDFLAGS has the right -L. On macOS, pipcl.build_extension() calls macos_patch2(path_so, libpaths, libs) after linking, and that fails:

  File "$SRC_DIR/setup.py", line 614, in build
    path_so_leaf = pipcl.build_extension(
  File ".../pipcl/__init__.py", line 2151, in build_extension
    macos_patch2(path_so, libpaths, libs)
  File ".../pipcl/__init__.py", line 2534, in macos_patch2
    for libpath in libpaths:
TypeError: 'NoneType' object is not iterable

This PR restores the 1.28.0 behaviour: with system MuPDF, libpaths comes from colon-separated PYMUPDF_MUPDF_LIB and libraries is None. Behaviour when mupdf_lib is set is unchanged.

Seen and verified in the conda-forge build of 2.0, which applies the same change as a patch: macOS builds went from failing to passing (conda-forge/pymupdf-feedstock#60).

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