Expat: build on older GLibC versions

getrandom() is only in GLibC >= 2.2.5, and CentOS 7 uses an older
GCC version, so add CMake logic to fallback to /dev/urandom as the
Expat entroy source.

3rdparty/expat/CMakeLists.txt

@@ -1,7 +1,21 @@
 if (CMAKE_SYSTEM_NAME STREQUAL "Linux")
-    set(HAVE_GETRANDOM YES)
+    check_cxx_source_compiles(
+        "#include <sys/random.h>
+        int main(void) {
+            return 0;
+        }
+        "
+        HAVE_GETRANDOM)
+
+    if (NOT HAVE_GETRANDOM)
+        # on older (GLibC < 2.25) fall back to /dev/urandom
+        set(XML_DEV_URANDOM 1)
+    else()
+        message(STATUS "Found <sys/random.h>")
+    endif()
 endif()
 
+
 configure_file (
     "${PROJECT_SOURCE_DIR}/3rdparty/expat/expat_config_cmake.in"
     "${PROJECT_BINARY_DIR}/3rdparty/expat/simgear_expat_config.h"

3rdparty/expat/expat_config_cmake.in

@@ -11,6 +11,7 @@
 #endif
 
 #cmakedefine HAVE_GETRANDOM
+#cmakedefine XML_DEV_URANDOM
 
 // we can assume > 10.7 macOS so always set this
 #ifdef __APPLE__