Build with $(CC) instead of the hardcoded cc.

diff -Nrup a/Makefile b/Makefile
--- a/Makefile	2020-11-28 15:04:19.000000000 +0100
+++ b/Makefile	2026-10-22 17:22:15.837058650 +0200
@@ -1,7 +1,7 @@
 PREFIX = /usr
 
 colorpicker: main.c
-	cc -o colorpicker main.c `pkg-config --libs --cflags x11`
+	$(CC) -o colorpicker main.c `pkg-config --libs --cflags x11`
 
 install: colorpicker
 	mkdir -p $(DESTDIR)$(PREFIX)/bin/