I try to generate your current stable library on my openSUSE Tumbleweed system.
I stumble on the following open issues.
elfring@Sonne:~/Projekte/OpalVoip/OPAL/3.12> LANG=C make
make: Entering directory `/home/elfring/Projekte/OpalVoip/OPAL/3.12/plugins/video/H.263-1998'
h263-1998.cxx: In member function 'virtual bool H263_Base_Encoder::SetOption(const char*, const char*)':
h263-1998.cxx:424:47: error: 'CODEC_FLAG_H263P_UMV' was not declared in this scope
h263-1998.cxx:430:47: error: 'CODEC_FLAG_OBMC' was not declared in this scope
h263-1998.cxx:447:47: error: 'CODEC_FLAG_H263P_SLICE_STRUCT' was not declared in this scope
h263-1998.cxx:452:47: error: 'CODEC_FLAG_H263P_AIV' was not declared in this scope
../common/ffmpeg.cxx: In member function 'virtual bool FFMPEGCodec::InitDecoder(AVCodecID)':
../common/ffmpeg.cxx:450:14: error: 'AVCodecContext' has no member named 'error_recognition'
../common/ffmpeg.cxx:450:34: error: 'FF_ER_AGGRESSIVE' was not declared in this scope
../common/ffmpeg.cxx: In member function 'virtual bool FFMPEGCodec::EncodeVideoPacket(const PluginCodec_RTP&, PluginCodec_RTP&, unsigned int&)':
../common/ffmpeg.cxx:653:72: error: 'FF_I_TYPE' was not declared in this scope
A couple of other software projects have also got the challenge to update their support for the current programming interface here.