mesa and libglvnd install the same headers
libglvnd installs GL/GLES/GLX/EGL header files since https://github.com/NVIDIA/libglvnd/commit/ab9b5fcc3bf90064418f6915cf4259fa11ffe64b which is included in yesterdays 1.2.0 release. mesa no longer installs the pkg-config files when libglvnd support is enabled but still installs those header files.
So in order to prevent collisions between mesa (19.2.0) and libglvnd (1.2.0) one has to either manually remove the headers installed by mesa in favour of the ones installed by libglvnd or build libglvnd with --disable-headers which isn't ideal either since it now provides pkg-config files for headers it doesn't install