Define _FORTIFY_SOURCE=3 & _GLIBCXX_ASSERTIONS that add glibc/libstdc++ assertions.

See https://www.gnu.org/software/libc/manual/html_node/Source-Fortification.html & https://gcc.gnu.org/wiki/LibstdcxxDebugMode

_GLIBCXX_ASSERTIONS is probably useless as we have pure C here, but let's define it in case some checks are included (or will be in a future version).
As we handle some requests here, that may help to trap a problem.

configure.ac

@@ -309,10 +309,10 @@ fi
 
 AC_ARG_ENABLE(assertions,
               AS_HELP_STRING([--enable-assertions],
-                             [Turn on assertions checks (undefine NDEBUG)]),
+                             [Turn on assertions checks (undefine NDEBUG, define _GLIBCXX_ASSERTIONS  & _FORTIFY_SOURCE)]),
 [
   if test "${enableval}" = "yes"; then
-      assertions='-UNDEBUG'
+      assertions='-UNDEBUG -D_FORTIFY_SOURCE=3 -D_GLIBCXX_ASSERTIONS'
   else
       assertions='-DNDEBUG'
   fi