xlnt-community / xlnt / 4b5fed6e-c4d0-4ab7-aaf5-fdf02e395ff5

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enable relocatable build for Mac (#129)

By default, setting `INSTALL_NAME_DIR` to `$XLNT_LIB_DEST_DIR` will
result in a relative path being baked into the `.dylib` file. This can
be verified with `otool -D xlnt.dylib`.

That means that when you try to to compile and run a test program, it
will fail at runtime:

```
dyld[25352]: Library not loaded: lib/libxlnt-1.6.1.dylib
...
  Reason: tried 'lib/libxlnt.1.6.1.dylib' (no such file) [...]
```

The solution that Apple likes is to use
[rpath](https://developer.apple.com/library/archive/documentation/DeveloperTools/Conceptual/DynamicLibraries/100-Articles/RunpathDependentLibraries.html),
creating a relocatable library, where the consumer uses the
`-Wl,-rpath,/some/prefix/lib` options and the dynamic linker will look
in that directory at runtime. A prefix using the string `@rpath` ends up
baked into the binary.

This means that, after installing xlnt with the default build options,
options to build the demo program would look like:

`-std=c++14 -Ixlnt/include -lxlnt -Wl,-rpath,/usr/local/lib`

Another way to fix the same problem would be to ensure that the value
for `INSTALL_NAME_DIR` was absolute rather than relative. Overall I'm
not sure which way is the best, but this one seems to be the
Apple-approved way.

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