Compile error in ffmpeg git master with opencv 3.4.1

[bermond]

I'm getting the following error at configure time when trying to compile ffmpeg git-master with the newly released opencv 3.4.1:

ERROR: libopencv not found

Relevant ffbuild/config.log section:

In file included from /usr/include/opencv2/core/types_c.h:59:0,
                 from /usr/include/opencv2/core/core_c.h:48,
                 from /tmp/ffconf.Rgo00uNd/test.c:1:
/usr/include/opencv2/core/cvdef.h:485:1: error: unknown type name 'namespace'
 namespace cv {
 ^~~~~~~~~
/usr/include/opencv2/core/cvdef.h:485:14: error: expected '=', ',', ';', 'asm' or '__attribute__' before '{' token
 namespace cv {
              ^
ERROR: libopencv not found

It compiles fine when using opencv 3.4.0.

How to reproduce:

$ git clone https://git.ffmpeg.org/ffmpeg.git
$ cd ffmpeg
$ ./configure --enable-libopencv

A relevant ​issue (#10963) is opened at the official opencv github repository regarding the same error when compiling vlc. Here is a quote of opencv developer Alexander Alekhin when answering this issue: "OpenCV 3.x doesn't not support C compilation mode officially. There are still legacy C API calls (from OpenCV 1.x) but they should be compiled in C++ compilation mode".

System information:
OS: Arch Linux x86_64
Compiler: gcc 7.3.0
FFmpeg: git master
OpenCV: 3.4.1

[joe]

version 3.0 is obsolete and not available from package managers anymore. It's 10 years old.
Maybe it's time to support v4.