tools: expat: fix build on older host systems

Expat release 2.2.2 requires support for either syscall(SYS_getrandom) which
is available on Linux 3.17 or support for getrandom() which is only available
in glibc 2.25 or later.

Since some of our builders still run on Linux 3.16, we need to forcibly
disable the use of getrandom() for the host builds.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

tools/expat/Makefile

@@ -18,6 +18,9 @@ HOST_BUILD_PARALLEL:=1
 
 include $(INCLUDE_DIR)/host-build.mk
 
+HOST_CONFIGURE_VARS += \
+	CPPFLAGS="$(HOST_CFLAGS) -DXML_POOR_ENTROPY"
+
 define Host/Install
 	$(MAKE) -C $(HOST_BUILD_DIR) install
 endef