doc/Makefile: invoke perl to interpret texi2pod.pl

Do not rely on shebang directive in the script, whose exact behavior is
system dependent.

doc/Makefile

@@ -43,7 +43,7 @@ doc/%.html: doc/%.texi $(SRC_PATH)/doc/t2h.init $(GENTEXI)
 doc/%.pod: TAG = POD
 doc/%.pod: doc/%.texi $(GENTEXI)
 	$(Q)$(TEXIDEP)
-	$(M)$(SRC_PATH)/doc/texi2pod.pl -Idoc $< $@
+	$(M)perl $(SRC_PATH)/doc/texi2pod.pl -Idoc $< $@
 
 doc/%.1: TAG = MAN
 doc/%.1: doc/%.pod $(GENTEXI)