makefile: rather than a dependency for each source file, generate loglevels.h by recursion

This little hack runs make recursively to generate include/loglevels.h
before running any other target but skip an explicit dependency on
loglevels.h in each rule. Therefore, you do not need to rebuild
every source file when compiling.

Makefile

@@ -8,12 +8,22 @@ FILES:=$(filter-out $(AUTOGENERATED),$(wildcard src/*.c))
 FILES:=$(FILES:.c=.o)
 HEADERS:=$(filter-out include/loglevels.h,$(wildcard include/*.h))
 
+# Recursively generate loglevels.h by explicitly calling make
+# We need this step because we need to ensure that loglevels.h will be
+# updated if necessary, but we also want to save rebuilds of the object
+# files, so we cannot let the object files depend on loglevels.h.
+ifeq ($(MAKECMDGOALS),loglevels.h)
+UNUSED:=$(warning Generating loglevels.h)
+else
+UNUSED:=$(shell $(MAKE) loglevels.h)
+endif
+
 # Depend on the specific file (.c for each .o) and on all headers
-src/%.o: src/%.c ${HEADERS} loglevels.h
+src/%.o: src/%.c ${HEADERS}
 	echo "CC $<"
 	$(CC) $(CFLAGS) -DLOGLEVEL="(1 << $(shell awk '/$(shell basename $< .c)/ { print NR }' loglevels.tmp))" -c -o $@ $<
 
-all: loglevels.h src/cfgparse.y.o src/cfgparse.yy.o ${FILES}
+all: src/cfgparse.y.o src/cfgparse.yy.o ${FILES}
 	echo "LINK i3"
 	$(CC) -o i3 ${FILES} src/cfgparse.y.o src/cfgparse.yy.o $(LDFLAGS)
 	echo ""
@@ -22,10 +32,7 @@ all: loglevels.h src/cfgparse.y.o src/cfgparse.yy.o ${FILES}
 	echo "SUBDIR i3-input"
 	$(MAKE) TOPDIR=$(TOPDIR) -C i3-input
 
-rm_loglevels:
-	rm -f loglevels.h
-
-loglevels.h: rm_loglevels
+loglevels.h:
 	echo "LOGLEVELS"
 	for file in $$(ls src/*.c src/*.y src/*.l | grep -v 'cfgparse.\(tab\|yy\).c'); \
 	do \
@@ -35,14 +42,14 @@ loglevels.h: rm_loglevels
 	do \
 		echo "\"$$file\", "; \
 	done; \
-	echo "};") > include/loglevels.h
+	echo "};") > include/loglevels.h;
 
-src/cfgparse.yy.o: src/cfgparse.l loglevels.h
+src/cfgparse.yy.o: src/cfgparse.l
 	echo "LEX $<"
 	flex -i -o$(@:.o=.c) $<
 	$(CC) $(CFLAGS) -DLOGLEVEL="(1 << $(shell awk '/cfgparse.l/ { print NR }' loglevels.tmp))" -c -o $@ $(@:.o=.c)
 
-src/cfgparse.y.o: src/cfgparse.y loglevels.h
+src/cfgparse.y.o: src/cfgparse.y
 	echo "YACC $<"
 	bison --debug --verbose -b $(basename $< .y) -d $<
 	$(CC) $(CFLAGS) -DLOGLEVEL="(1 << $(shell awk '/cfgparse.y/ { print NR }' loglevels.tmp))" -c -o $@ $(<:.y=.tab.c)

common.mk

@@ -74,5 +74,5 @@ endif
 .SILENT:
 
 # Always remake the following targets
-.PHONY: install clean dist distclean rm_loglevels
+.PHONY: install clean dist distclean