Pull up following revision(s) (requested by maya in ticket #1394):

external/mit/expat/lib/libexpat/Makefile: revision 1.3
external/mit/expat/lib/libexpat/Makefile: revision 1.4

Now that expat_config.h uses a header that is OS-specific, using the HOST_CC
breaks for some setups which lack it (netbsd<8, possibly other things).

Use the compiler targetting netbsd that uses netbsd headers to figure out
PACKAGE_VERSION to avoid this problem.

Unless we add --sysroot, the netbsd src tree headers won't be used.
Do so, so we don't rely on the build machine to have <endian.h> for
this macro check.

Using ${CPPFLAGS} rather than my own encoding of --sysroot, since it
already includes it.

Tested by agc, thanks.

(martin)