On a Gentoo system using the stable x86 branch, emerging XDrawChem 1.7.5 will fail during build configuration if OpenBabel 1.100.2 (available in the stable x86 branch) is installed. The reason is that XDrawChem 1.7.5 can only be compiled against version 1.100.1 of OpenBabel, while the ebuild for version 1.7.5 of XDrawChem uses : >=app-sci/openbabel-1.100.1*. I suggest changing the dependency to =app-sci/openbabel-1.100.1* or bumping XDrawChem 1.7.8 to the stable x86 branch.
*** Bug 45911 has been marked as a duplicate of this bug. ***
Done, thanks for the hint.