expat: fix build on and for kernel older than 3.17

expat: fix build on and for kernel older than 3.17

The expat build system now fails when the getrandom() system call is not
supported. This affect both host and target builds. Define XML_POOR_ENTROPY
for target kernels older than 3.17 to fix the build. For the host package
define XML_POOR_ENTROPY unconditionally since we have no easy way to know the
host kernel version. Note that expat will still use getrandom() on the host
when it is available, we don't make security any worse.

Fixes (host):
http://autobuild.buildroot.net/results/928/928dc2b56d931da84055fdfe78929d1f956de53b/
http://autobuild.buildroot.net/results/ee9/ee90d0a456cbce4c7f22e5f61006612bd9ba30d5/
http://autobuild.buildroot.net/results/dac/dac7231242123ae3dcaa6bbdd65b44fe8d8cb20c/

Fixes (target):
http://autobuild.buildroot.net/results/308/308e830219fdfebb5aa6aef51c1dc784254998f6/
http://autobuild.buildroot.net/results/73f/73fa946b0a2205e946ad414079f88e4bdb416f00/
http://autobuild.buildroot.net/results/9d7/9d7bad22ace7fa211b31d752a2255e07cede68be/

[Peter: also use HOST_CPPFLAGS]
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>