hdf4: fix invalid rpath on darwin
hdf4: fix invalid rpath on darwin

CMake changes in hdf4 v4.15.2 broke the library path on macOS,
linking using an invalid rpath rather than an absolute path.

Before this commit:
```
otool -L result/lib/libhdf.dylib
result/lib/libhdf.dylib:
	@rpath/libhdf.4.dylib (compatibility version 4.0.0, current version 4.15.2)
```

After:
```
otool -L result/lib/libhdf.dylib
result/lib/libhdf.dylib:
	/nix/store/bz52b2gwci0k8rwd0llsi555s1hx166j-hdf-4.2.15/lib/libhdf.4.15.2.dylib (compatibility version 4.0.0, current version 4.15.2)
```

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