Ffmpeg
ffmpeg is a set of video processing tools used by ZoneMinder to generate true video files from the frames captured and saved. It has always been under heavy development.
Under ffmpeg 0.4.8, the directions in the official ZoneMinder documentation are exactly right - use './configure <extra options>' followed by 'make', 'make install', and 'make installib' - to compile and install the software for use with ZoneMinder.
Under the snapshots at least as old as 2008-09-25, the installlib make target has been removed. Instead, it's done in the configure stage with './configure --enable-shared --enable-swscale --enable-gpl <extra options>'. Without passing these extra options to configure, ZoneMinder will be unable to find the header files that it needs to compile correctly.
I am unsure as to how common this problem is, but my Slackware box came with GNU make 3.80. You can check your version with 'make -L'. The recent development snapshots of ffmpeg will choke with this version of make with a 'virtual memory exhausted' error message. The solution is to update the version of GNU make on your system to at least 3.81. (This, in my opinion, is a bug in ffmpeg that should be remedied; thankfully, the solution is simple enough.)