feat(rules): Allow frontmatter to qualify a source as having metadata

Author: alerque

rules/rules.mk

@@ -25,9 +25,10 @@ MARKDOWNSOURCES := $(patsubst ./%,%,$(call find,*.md))
 LUASOURCES := $(patsubst ./%,%,$(call find,*.lua))
 MAKESOURCES := $(patsubst ./%,%,$(call find,[Mm]akefile)) $(patsubst ./%,%,$(call find,*.mk))
 YAMLSOURCES := $(patsubst ./%,%,$(call find,*.yml)) $(patsubst ./%,%,$(call find,*.yaml))
+METASOURCES := $(shell $(_ENV) $(RG) -lFxe --- $(MARKDOWNSOURCES))
 
 # Find stuff that could be built based on what has matching YAML and a MD components
-SOURCES_DEF := $(filter $(basename $(notdir $(MARKDOWNSOURCES))),$(basename $(notdir $(YAMLSOURCES))))
+SOURCES_DEF := $(filter $(basename $(notdir $(MARKDOWNSOURCES))),$(basename $(notdir $(YAMLSOURCES) $(METASOURCES))))
 SOURCES ?= $(SOURCES_DEF)
 TARGETS ?= $(SOURCES)