The Makefiles should put binaries in $(EXEC_PREFIX) and
architecture-independent files in $(PREFIX). Also a user may have a
prefixed- pkg-config, as in the case of cross compiling on Exherbo
Linux, so respect the well-accepted $(PKG_CONFIG) variable for this
purpose.
CPPFLGES, CFLAGS and LDFLAGS should be user variables
We now provide default flags but use I3_*FLAGS flags for our own needed
flags
Also reoder lib flags a bit
