[SOLVED] XDM fails to start KDE desktop (NVIDIA)

[libfab]

There seemingly is an OpenGL incompatibilty between the new mesa ebuilds (version 12.01 recentlly sync'ed) and NVIDIA drivers.
I run a GTX 970 Nvidia card and driver is the latest from proprietary installer (NVIDIA-Linux-x86_64-375.39.run downloaded from NVIDIA's website)
Installing gentoo nvidia-drivers did not solve the issue, even by closely following the gentoo manual dedicated page.

Platform is stable branch amd64 built against g++-5.4.

Reverting to mesa 11.2.2 solved the issue completely without having to rebuild any external dependency or apps, so the issue is strictly mesa-related.

I just wanted to know if there were known ways out before possibly filing a bug report.

[libfab]

Issue was solved by rebuilding all gcc-related dependencies.
I conjecture that there may have been reverse dependencies that should have been rebuilt after the new mesa install yet remained undetected by portage and revdep-rebuild.