build: enable -Wdocumentation if suppressing external warnings

Co-authored-by: Ben Woosley <ben.woosley@gmail.com>

configure.ac

@@ -439,6 +439,10 @@ if test "x$enable_werror" = "xyes"; then
                         [AC_LANG_SOURCE([[struct A { virtual void f(); }; struct B : A { void f() final; };]])])
   AX_CHECK_COMPILE_FLAG([-Werror=unreachable-code-loop-increment],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=unreachable-code-loop-increment"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Werror=mismatched-tags], [ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=mismatched-tags"], [], [$CXXFLAG_WERROR])
+
+  if test x$suppress_external_warnings != xno ; then
+    AX_CHECK_COMPILE_FLAG([-Werror=documentation],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=documentation"],,[[$CXXFLAG_WERROR]])
+  fi
 fi
 
 if test "x$CXXFLAGS_overridden" = "xno"; then
@@ -466,6 +470,10 @@ if test "x$CXXFLAGS_overridden" = "xno"; then
                         [AC_LANG_SOURCE([[struct A { virtual void f(); }; struct B : A { void f() final; };]])])
   AX_CHECK_COMPILE_FLAG([-Wunreachable-code-loop-increment],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wunreachable-code-loop-increment"],,[[$CXXFLAG_WERROR]])
 
+  if test x$suppress_external_warnings != xno ; then
+    AX_CHECK_COMPILE_FLAG([-Wdocumentation],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wdocumentation"],,[[$CXXFLAG_WERROR]])
+  fi
+
   dnl Some compilers (gcc) ignore unknown -Wno-* options, but warn about all
   dnl unknown options if any other warning is produced. Test the -Wfoo case, and
   dnl set the -Wno-foo case if it works.