Building Sqlite3 with R*Tree on OSX

I am trying to build SQLite with R*tree module on OSX. Here are some relevant parts of my makefile

CFLAGS= -O1 -DSQLITE_ENABLE_RTREE=1
SFLAGS= -ldl -lpthread 
##############################################################################
%.o: %.c
    gcc -c $(CFLAGS) $< -o $@

#Building SQLite3 shell for administrative purposes
shell : shell.c sqlite3.o
    gcc $(SFLAGS) -o shell.out shell.c sqlite3.o 

However, the RTree is still not available. I do have a copy of sqlite3 installed using homebrew and RTree works fine on that. However, I am not sure what is wrong when I build the amalgamation myself.