Use "command -v" instead of "which"
The behavior of "which" is not standardized by POSIX. Some old implementations print their error messages to stdout instead of stderr, and don't return a nonzero exit code when they fail to find the given program. "command -v" on the other hand is in POSIX (optional in 2004 and required as of 2008).
Remove otherwise-unused variables AUTORECONF and GTKDOCIZE.