Add 'order-only-prerequisites' to make doc builds deterministic

This ensure that the docs are built in the same order.

Fixes: #89

Makefile.am

@@ -95,13 +95,13 @@ docs/reference/version.entities:
 docs-bt-core: libbuzztrax-core.la libbtbsl.la libbuzztrax-ic.la docs/reference/version.entities
 	@$(MAKE) -C docs/reference/bt-core
 
-docs-bt-ic: libbuzztrax-ic.la docs/reference/version.entities
+docs-bt-ic: libbuzztrax-ic.la docs/reference/version.entities | docs-bt-core 
 	@$(MAKE) -C docs/reference/bt-ic
 
-docs-bt-cmd: libbtcmd.la libbuzztrax-ic.la libbuzztrax-core.la docs/reference/version.entities
+docs-bt-cmd: libbtcmd.la libbuzztrax-ic.la libbuzztrax-core.la docs/reference/version.entities | docs-bt-core docs-bt-ic
 	@$(MAKE) -C docs/reference/bt-cmd
 
-docs-bt-edit: libbtedit.la libbuzztrax-ic.la libbuzztrax-core.la docs/reference/version.entities
+docs-bt-edit: libbtedit.la libbuzztrax-ic.la libbuzztrax-core.la docs/reference/version.entities | docs-bt-core docs-bt-ic
 	@$(MAKE) -C docs/reference/bt-edit
 
 if FLUIDSYNTH_SUPPORT