Looks like config.h.meson is missing to build gst-omx with meson.

$ meson build/ -Dwith_omx_target=generic
The Meson build system
Version: 0.40.0
Source dir: /home/cassidy/dev/gst/master/gst-build/subprojects/gst-omx
Build dir: /home/cassidy/dev/gst/master/gst-build/subprojects/gst-omx/build
Build type: native build
Project name: gst-omx
Native c compiler: cc (gcc 6.3.1)
Build machine cpu family: x86_64
Build machine cpu: x86_64
Found pkg-config: /usr/bin/pkg-config (0.29.1)
Native dependency gstreamer-1.0 found: YES 1.13.0.1
Native dependency gstreamer-base-1.0 found: YES 1.13.0.1
Native dependency gstreamer-controller-1.0 found: YES 1.13.0.1
Native dependency gstreamer-pbutils-1.0 found: YES 1.13.0.1
Native dependency gstreamer-audio-1.0 found: YES 1.13.0.1
Native dependency gstreamer-fft-1.0 found: YES 1.13.0.1
Native dependency gstreamer-tag-1.0 found: YES 1.13.0.1
Native dependency gstreamer-video-1.0 found: YES 1.13.0.1
Native dependency gstreamer-gl-1.0 found: YES 1.13.0.1
Library m found: YES
Native dependency glib-2.0 found: YES 2.50.3
Native dependency gio-2.0 found: YES 2.50.3
Native dependency gmodule-2.0 found: YES 2.50.3
Has header "OMX_Core.h": YES
Has header "OMX_VideoExt.h": NO
Header <OMX_Video.h> has symbol "OMX_VIDEO_CodingVP8": NO
Header <OMX_Video.h> has symbol "OMX_VIDEO_CodingTheora": NO
Configuring config.h using configuration

Meson encountered an error in file meson.build, line 248, column 0:
Could not read input file /home/cassidy/dev/gst/master/gst-build/subprojects/gst-omx/config.h.meson: [Errno 2] No such file or directory: '/home/cassidy/dev/gst/master/gst-build/subprojects/gst-omx/config.h.meson'