Megatools dev doesn't state what GCC version is the minimum required
for building, but they imply at least 5.4 should be used:

https://github.com/megous/megatools/issues/381

Thus, add USES=compiler:c11, since the port doesn't build with base GCC.

PR: 233592
Submitted by: Piotr Kubaj
Approved by: portmgr (tier-2 blanket)