configure: Warn or error (as appropriate) if icon rendering tools are missing

Author: luke-jr

configure.ac

@@ -779,6 +779,12 @@ case $host in
 esac
 
 AM_CONDITIONAL([CAN_RENDER_ICONS], [test "$can_render_icons" = "yes"])
+if test "$can_render_icons" != "yes"; then
+  if test ! -f "${srcdir}/src/qt/res/rendered_icons/bitcoin.ico"; then
+    AC_MSG_ERROR([Couldn't find ${can_render_icons} (required to build from git)])
+  fi
+  AC_MSG_WARN([Couldn't find ${can_render_icons}; you won't get the Knots-branded Bitcoin icon])
+fi
 
 if test "$use_extended_functional_tests" != "no"; then
   AC_SUBST(EXTENDED_FUNCTIONAL_TESTS, --extended)