On compiling the latest Git, against up-to-date Git FFmpeg using your new FFmpeg linkage, there is a type mismatch in the argument _output.c_str() passed to function avio_open (&_format_context->pb, _output.c_str(), AVIO_FLAG_WRITE) in line 162 of src/lib/ffmpeg_encoder.cc

This is compilation under Mingw-w64 gcc (under Ubuntu Bash on Windows) and g++ version 6.3.1

FFmpeg build: github dated 11th May 2017 (today)
dcpomatic version: github dated 11th May 2017 (today)

g++ reports via waf:

[ 70/287] Compiling src/lib/hints.cc
[ 71/287] Compiling src/lib/internet.cc
../src/lib/ffmpeg_encoder.cc: In member function 'virtual void FFmpegEncoder::go()':
../src/lib/ffmpeg_encoder.cc:162:71: error: cannot convert 'const value_type {aka const wchar_t}' to 'const char*' for argument '2' to 'int avio_open(AVIOContext*, const char, int)'
if (avio_open (&_format_context->pb, _output.c_str(), AVIO_FLAG_WRITE) < 0) {
^

Waf: Leaving directory `/mnt/e/Users/john/Documents/MultimediaTools-mingw-w64/sandbox/x86_64/dcpomatic/build'
Build failed
-> task in 'libdcpomatic2' failed (exit status 1):
{task 140118553837840: cxx ffmpeg_encoder.cc -> ffmpeg_encoder.cc.1.o}
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Build failure. Please see error messages above.