[source] tools: expat: fix build on older host systems

[LEDE Commits]

jow pushed a commit to source.git, branch master:
https://git.lede-project.org/7c727c6fa427dba9e43aaf3648d12af19a4858f1

commit 7c727c6fa427dba9e43aaf3648d12af19a4858f1
Author: Jo-Philipp Wich <jo at mein.io>
AuthorDate: Tue Jul 18 22:50:11 2017 +0200

    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 at mein.io>
---
 tools/expat/Makefile | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tools/expat/Makefile b/tools/expat/Makefile
index 697d3f6..246089d 100644
--- a/tools/expat/Makefile
+++ b/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