When Makefile.am is modified, make sure custom-entities.ent is rebuilt.
After all, $(substitutions) is defined there, so changes of that variable
must be reflected in the resulting file.
# ------------------------------------------------------------------------------
if ENABLE_MANPAGES
-man/custom-entities.ent: configure.ac
+man/custom-entities.ent: Makefile.am configure.ac
$(AM_V_GEN)$(MKDIR_P) $(dir $@)
$(AM_V_GEN)(echo '<?xml version="1.0" encoding="utf-8" ?>' && \
printf '$(subst '|,<!ENTITY ,$(subst =, ",$(subst |',">\n,$(substitutions))))') \