ffmpeg was fine with g++. Turns out the issue was specifically with

boost building with c++11 abi turned on.

ffmpeg/Makefile

@@ -16,12 +16,6 @@ ffmpeg_impl:
 	sudo apt-get install -y yasm
 	sudo apt-get install -y nasm # apparently x264 switched to this
 
-	update-alternatives --install /usr/bin/c++ c++ /usr/bin/clang++-7 40
-	update-alternatives --install /usr/bin/cc cc /usr/bin/clang-7 40
-	update-alternatives --install /usr/bin/clang++ clang++ /usr/bin/clang++-7 40
-	update-alternatives --install /usr/bin/clang clang /usr/bin/clang-7 40
-	update-alternatives --install /usr/bin/ld ld /usr/bin/ld.lld-7 40
-
 	mkdir x264_out
 	mkdir ffmpeg_out