Makefile: avoid spurious rebuilds of git sources due to `sudo make`

Signed-off-by: Lars Hjemli <hjemli@gmail.com>
diff --git a/Makefile b/Makefile
index 15c7c20..be1d796 100644
--- a/Makefile
+++ b/Makefile
@@ -126,6 +126,8 @@
 CFLAGS += -DCGIT_SCRIPT_NAME='"$(CGIT_SCRIPT_NAME)"'
 CFLAGS += -DCGIT_CACHE_ROOT='"$(CACHE_ROOT)"'
 
+GIT_OPTIONS = prefix=/usr
+
 ifdef NO_ICONV
 	CFLAGS += -DNO_ICONV
 endif