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 https://github.com/boostorg/geometry/issues/1311.
