ffmpeg issue building on macOS

[advicepyro]

Describe the bug

There appears to be issues doing a build with build-debug on macOS. This is assuming I want ffmpeg support.

Without ffmpeg installed:

ld: library not found for -lavcodec
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Installing ffmpeg from homebrew, build fails twice at:

hardware.cpp:422:27: error: use of undeclared identifier 'CODEC_FLAG2_FAST'
        ffmpeg_vid_ctx->flags2 = CODEC_FLAG2_FAST;

hardware.cpp:1030:29: error: use of undeclared identifier 'CODEC_FLAG2_FAST'
                        ffmpeg_vid_ctx->flags2 = CODEC_FLAG2_FAST;

I am able to compile DOSBox-X using build-debug-noavcodec and then run the binary from /usr/bin/dosbox-x after make install.

To Reproduce

Steps to reproduce the behavior:

1. Checkout this project at the given commit.
2. Install latest version of Xcode, and Homebrew.
3. Install autoconf, automake, ffmpeg from Homebrew.
4. Run build-debug

Expected behavior

Expected compile to complete without errors.

Environment (please complete the following information):

• Operating system: macOS 10.13.5, Xcode 9.4.1
• DOSBox-X commit 6d06104
• Installed homebrew packages and versions:

autoconf 2.69
automake 1.16.1
coreutils 8.29 8.30
cython 0.28.4
faac 1.29.9.2
fdk-aac 0.1.6
ffmpeg 4.0.1
fluid-synth 1.1.11
freetype 2.9.1
gdbm 1.16
gettext 0.19.8.1
glib 2.56.1
gti 1.6.1
htop 2.2.0
lame 3.100
libffi 3.2.1
libogg 1.3.3
libpng 1.6.34
libvo-aacenc 0.1.3
libvorbis 1.3.6
libvpx 1.7.0
makedepend 1.0.5
openssl 1.0.2o_2
opus 1.2.1
pcre 8.42
pkg-config 0.29.2
python@2 2.7.15_1
readline 7.0.5
sdl 1.2.15
sdl_net 1.2.8
sdl_sound 1.0.3_1
sl 5.02
sleepwatcher 2.2
sphinx-doc 1.7.5_1
sqlite 3.24.0
texi2html 5.0
x264 r2854
xvid 1.3.5
yasm 1.3.0_1

[dlitz]

CODEC_FLAG2_FAST should be replaced with AV_CODEC_FLAG2_FAST in hardware.cpp