Fix doxygen_xml2qbk execution from make_qbk.py

[Lastique]

When b2 invokes doc/make_qbk.py, it passes a relative path to the compiled doxygen_xml2qbk executable. This path becomes invalid if used from a directory different from doc, which may happen as make_qbk.py changes the current directory during its execution. Additionally, doc/index/make_qbk.py invokes doxygen_xml2qbk unqualified, which requires the executable to be in PATH and is never the case unless the user has pre-compiled and placed it accordingly.

To fix this, first resolve the paths to doxygen and doxygen_xml2qbk to absolute paths before changing the current directory. Additionally, pass the resolved path to doxygen_xml2qbk to doc/index/make_qbk.py via the DOXYGEN_XML2QBK environment variable.

Fixes #1311.

[barendgehrels]

Thanks a lot!
So this fixes a structural issue and makes making documentation easier.

For others, @Lastique also solved the "1.79.2" problem, as discussed here
boostorg/boostbook#13

For that research, this PR was essential.

[vissarion]

Thanks a lot, looks good to me, I approve.

Just a note on documentation, there is a CI job that builds documentation and seems to work as expected. Of course it is on Ubuntu so it cannot catch issues on Mac/win etc.

[Lastique]

The documentation CI workflow works because it explicitly builds doxygen_xml2qbk and copies it to /usr/local/bin prior to building the documentation. This is unconventional and should not be required. This PR fixes this, and building docs should now work by simply invoking b2 in libs/geometry/doc. It may be a good idea to update the CI workflow to verify that.

[vissarion]

The documentation CI workflow works because it explicitly builds doxygen_xml2qbk and copies it to /usr/local/bin prior to building the documentation. This is unconventional and should not be required. This PR fixes this, and building docs should now work by simply invoking b2 in libs/geometry/doc. It may be a good idea to update the CI workflow to verify that.

That is a good suggestion, I opened a PR. Please feel free to review it.