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

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

Makefile

@@ -126,6 +126,8 @@ CFLAGS += -DCGIT_CONFIG='"$(CGIT_CONFIG)"'
 CFLAGS += -DCGIT_SCRIPT_NAME='"$(CGIT_SCRIPT_NAME)"'
 CFLAGS += -DCGIT_CACHE_ROOT='"$(CACHE_ROOT)"'
 
+GIT_OPTIONS = prefix=/usr
+
 ifdef NO_ICONV
 	CFLAGS += -DNO_ICONV
 endif