Allow user to select temperature unit

Makefile

@@ -7,7 +7,7 @@ LD=clang
 LDFLAGS=-lgpio -lfigpar -lpthread ${SQLITE3_LDFLAGS}
 SRCS=src/main.c src/util.c src/lcd.c src/ths.c src/button.c src/ui/screen.c\
      src/ui/datesel.c src/ui/statsby.c src/ui/datapoints.c src/ui/timesel.c\
-     src/ui/statrange.c
+     src/ui/statrange.c src/config.c
 PROG=rpi4b-temp-humidity
 
 OBJS=${SRCS:C/^src/bin/:C/.c$/.o/}
@@ -29,17 +29,22 @@ bin/ui/statrange.o: src/ui/screen.h
 bin/main.o bin/ui/datesel.o bin/ui/statsby.o: src/ui/datesel.h
 bin/ui/datapoints.o bin/ui/timesel.o bin/ui/statrange.o: src/ui/datesel.h
 
-bin/main.o bin/ths.o bin/ui/screen.o bin/ui/statsby.o: src/ths.h
-bin/ui/timesel.o src/ui/statrange.o: src/ths.h
 
 bin/main.o bin/ui/timesel.o bin/ui/datapoints.o: src/ui/timesel.h
 bin/ui/statrange.o: src/ui/timesel.h
 
+bin/main.o bin/ths.o: src/ths.h
+bin/main.o bin/config.o: src/config.h
 bin/main.o bin/button.o bin/ui/screen.o: src/button.h
 bin/main.o bin/ui/statsby.o: src/ui/statsby.h
 bin/main.o bin/ui/datapoints.o: src/ui/datapoints.h
 bin/main.o bin/ui/statrange.o: src/ui/statrange.h
 
+.if "${DEFAULT_TEMP_UNIT}" != "F" && "${DEFAULT_TEMP_UNIT}" != "C" &&\
+    "${DEFAULT_TEMP_UNIT}" != "c" && "${DEFAULT_TEMP_UNIT}" != "c"
+.error Invalid temperature unit "${DEFAULT_TEMP_UNIT}"
+.endif
+
 ${OBJS}: ${.TARGET:C/^bin/src/:C/.o$/.c/}
 	@mkdir -p ${.TARGET:H}
 	${CC} ${CFLAGS} -c\
@@ -51,6 +56,7 @@ ${OBJS}: ${.TARGET:C/^bin/src/:C/.o$/.c/}
 -DDEFAULT_FAIL_LIMIT="${DEFAULT_FAIL_LIMIT}"\
 -DDEFAULT_DATABASE_LOCATION="\"${DEFAULT_DATABASE_LOCATION}\""\
 -DDEFAULT_REFRESH_TIME="${DEFAULT_REFRESH_TIME}"\
+-DDEFAULT_TEMP_UNIT="'${DEFAULT_TEMP_UNIT}'"\
 -o ${@} ${.TARGET:C/^bin/src/:C/.o$/.c/}
 
 clean: