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>

(cherry picked from commit 5242701f3aa8426cdf2cc9a176ef06194db93d5f)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/expat/expat.mk

@@ -15,5 +15,14 @@ HOST_EXPAT_DEPENDENCIES = host-pkgconf
 EXPAT_LICENSE = MIT
 EXPAT_LICENSE_FILES = COPYING
 
+# Kernel versions older than 3.17 do not support getrandom()
+ifeq ($(BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_17),)
+EXPAT_CONF_ENV += CPPFLAGS="$(TARGET_CPPFLAGS) -DXML_POOR_ENTROPY"
+endif
+
+# Make build succeed on host kernel older than 3.17. getrandom() will still
+# be used on newer kernels.
+HOST_EXPAT_CONF_ENV += CPPFLAGS="$(HOST_CPPFLAGS) -DXML_POOR_ENTROPY"
+
 $(eval $(autotools-package))
 $(eval $(host-autotools-package))