There is no real reason to disable glesv1 so add it to the build.
While here add a USE_GL for it.
Can you double check that graphics/libepoxy doesn't need updating as well. Just build it in poudriere with this change to mesa and see if it works.
Looking good otherwise.
Also, portmgr is blocking review because changes to Mk/, but Mk/Uses/gl.mk is x11@ responsibility, so it should be OK.
libepoxy is ok.
Why did you though there could be a problem ?
Libepoxy is some glue library, and it had an explicit dep on glesv2, so I thought it might do things with glesv1 as well. I just wanted to be sure.