FcConfigSetSysRoot - Set the system root directory
#include <fontconfig/fontconfig.h>
void FcConfigSetSysRoot (FcConfig *config, const FcChar8 *sysroot);
Set 'sysroot' as the system root directory. fontconfig prepend 'sysroot' to the
  cache directories in order to allow people to generate caches at the build
  time. Note that this causes changing current config. i.e. this function calls
  FcConfigSetCurrent() internally.