pkgconfig problem on RHEL-5

Hi, I'd like to ask ... I am trying to compile ffmpeg plugin for qmmp, but cmake fails to recognize the needed libraries: ... snip ... -- checking for modules 'libavcodec;libavformat' -- package 'libavcodec' not found -- package 'libavformat' not found ... snip ... The relevant CMakeLists.txt line is: pkg_check_modules(FFMPEG libavcodec libavformat) I've also tried to run pkg-config on commandline: [root@dhcp-lab-227 rhel]# pkg-config libavcodec; echo $? 1 [root@dhcp-lab-227 rhel]# pkg-config libavformat; echo $? 1 The same commands return zeros if run on Fedora 10 (with pkgconfig version 0.23-3.fc10). The libavcodec.pc and libavformat.pc files have the same contents both on RHEL (ffmpeg-devel-0.4.9-0.50.20080908.el5) and Fedora (ffmpeg-devel-0.4.9-0.52.20080908.fc10). I am a bit confused ... is it a bug? Is it just a version incompatibility? Any suggestions how to workaround it? - TIA K.