-MAKEINFO = env LANG= LC_MESSAGES= LC_ALL= LANGUAGE= makeinfo
-TEXI2HTML = $(MAKEINFO) --no-split --reference-limit=2000 --html
+lang_env = env LANG= LC_MESSAGES= LC_ALL= LANGUAGE=
+makeinfo_prog = makeinfo
+MAKEINFO = $(lang_env) $(makeinfo_prog)
+
+manual_opts = --no-split --reference-limit=2000
+TEXI2HTML = $(MAKEINFO) $(manual_opts) --html
+
+# Needed so that tex finds a recent enough texinfo.tex.
+tex_opts = -I ../build-aux