The fix in 0fb294a8 was only partially
valid, as it unconditionally used $includedir, without any prefix.  This
polluted the include path with host include files in cross-compiling
builds.

Let's take a different approach and simply define LITECELL15_CFLAGS
(similar to what pkgconfig does), which makes the "-I" go away if no
--with-litecell15 has been specified.

Change-Id: I63393decfe42a24dab56c7654f716c1580416ab2