Modify Makefile for adding MACRO of CROSS_COMPILE and modify gitignore.

.gitignore

 ### Temp files ###
 *.*~
+*.o
+timemanager
 
 ### VS Code ###
 /.vscode/

Makefile

-CC = gcc
-CFLAGS = `pkg-config --cflags gtk+-3.0`
-LIBS = `pkg-config --libs gtk+-3.0`
+# Set CROSS_COMPILE
+CROSS_COMPILE ?=
 
+# Make variables (CC, etc...)
+CC            = $(CROSS_COMPILE)gcc
+CFLAGS        = `pkg-config --cflags gtk+-3.0`
+LIBS          = `pkg-config --libs gtk+-3.0`
+
+# Objects
+objects       = main.o
+
+# Default target
 all: timemanager
 
-timemanager: main.c
+# Build objects
+timemanager: $(objects)
+
+# Temp objects
+%.o: %.c
 	$(CC) $(CFLAGS) $< -o $@ $(LIBS)
 
+# Clean
 .PHONY: clean
 clean:
-	rm *.o timemanager
+	rm -f *.o timemanager