The most common cause of this error is QuakeForge is unable to find pak0.pak. QuakeForge looks for pak0.pak in both the $fs_userpath/id1 and $fs_sharepath/id1 directories. In Windows, both fs_userpath and fs_sharepath default to "." (ie, the current directory: the same as id's Quake clients). In Linux (and other UNIX like operating systems), fs_userpath defaults to ~/.quakeforge and fs_sharepath defaults to $prefix/share/games/quakeforge (distribution packages might alter the exact path).
See QuakeForge Directory Trees for more details.
While QuakeForge itself (and the Quake source code in general) is Free Software, the game itself is not free. You need to either purchase it from id Software or you can use OpenQuartz, a project developing GPL-compatible game data for Quake.
Unlike downloading and compiling a release, when you checkout from git, you must have all the dependencies of QuakeForge already installed as when configure is being created autotools sources m4 files... If the files do not exist, you get the errors you have seen. Try installing packages which contain the libraries that caused the syntax error, including the -dev versions, then recreate configure using bootstrap and try again.
By default, QuakeForge no longer uses the CD drive to play music. Instead, QuakeForge now looks for a Tracklist config file to map Quake's cd tracks to sound files. If you wish to continue using the CD instead, you may set the
cd_plugin cvar to the value appropriate for your system:
This plugin can be used on most systems.
fileplugin. Not always available, and expects xmms to be available (but not necessarily running) on the system. Useful if you absolutely must have MP3 support.
Because they are not free. MP3 support is a patent minefield, and we really don't want to wind up with lawyers at ten paces, especially since we'd be unarmed with no armor, and they have rocket launchers and quad-damage.
Read-only cvars can be set by creating them before the engine does. See Read-only cvars for details.