The problem isn't GCC, it's that libavformat just updated and a variable name changed. Here's a diff:
*** makemkv-oss-1.12.2/libffabi/src/ffabi.c.ORIG 2018-04-27 20:53:47.000000000 -0500
--- makemkv-oss-1.12.2/libffabi/src/ffabi.c 2018-05-13 09:11:26.215809333 -0500
***************
*** 517,523 ...
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- Tue Jun 26, 2018 6:05 am
- Forum: MakeMKV for Linux
- Topic: Build failure with GCC 8? (Fedora 28)
- Replies: 6
- Views: 22100