well, it "works" by now, as in - it detects my non-std X11 installation prefix. documentation I was reading: https://autotools.io/pkgconfig/pkg_check_modules.html Problem is - for making it work that far I was forced to rename some PKG_* names from configure.ac into PKG1_* .... otherwise autoconf was complaining loudly about undefined macro: configure:6226: error: possibly undefined macro: PKG_FREETYPE configure:6288: error: possibly undefined macro: PKG_LZMA configure:6350: error: possibly undefined macro: PKG_PNG configure:6412: error: possibly undefined macro: PKG_PTHREAD configure:6474: error: possibly undefined macro: PKG_Z configure:6721: error: possibly undefined macro: PKG_LINUX_CDROM configure:6747: error: possibly undefined macro: PKG_LINUX_SYS configure:8780: error: possibly undefined macro: PKG_VDPAU configure:8846: error: possibly undefined macro: PKG_VAAPI configure:9101: error: possibly undefined macro: PKG_GL configure:9358: error: possibly undefined macro: PKG_XV configure:9395: error: possibly undefined macro: PKG_OSS configure:9423: error: possibly undefined macro: PKG_XFT configure:9530: error: possibly undefined macro: PKG_ALSA configure:9973: error: possibly undefined macro: PKG_OGG configure:10331: error: possibly undefined macro: PKG_FIREWIRE configure:10446: error: possibly undefined macro: PKG_DV configure:10484: error: possibly undefined macro: PKG_DVB configure:10772: error: possibly undefined macro: PKG_ESOUND configure:10969: error: possibly undefined macro: PKG_PULSE configure:11013: error: possibly undefined macro: PKG_ISOFS configure:11058: error: possibly undefined macro: PKG_LADSPA configure:11105: error: possibly undefined macro: PKG_SHUTTLE configure:11213: error: possibly undefined macro: PKG_SHUTTLE_USB configure:11646: error: possibly undefined macro: PKG_CUDA configure:11674: error: possibly undefined macro: PKG_DL configure:11760: error: possibly undefined macro: PKG_OPENCV configure:11781: error: possibly undefined macro: PKG_NUMA configure:11980: error: possibly undefined macro: PKG_OPENEXR yes, even from just trying ./autogen.sh I wonder how this problem must be solved, correctly?