gcc15 AVR compilation fixes (#25238)

builddefs/support.mk

@@ -0,0 +1,11 @@
+# Helper to determine if a compiler option is supported
+# Args:
+#   $(1) = option to test, if successful will be output
+#   $(2) = option to use if $(1) is not supported
+#   $(3) = additional arguments to pass to the compiler during the test, but aren't contained in the output
+cc-option = $(shell \
+	if { echo 'int main(){return 0;}' | $(CC) $(1) $(3) -o /dev/null -x c /dev/null >/dev/null 2>&1; }; \
+	then echo "$(1)"; else echo "$(2)"; fi)
+
+# Helper to pass comma character to make functions (use with `$(,)` to pass in `$(call ...)` arguments)
+, := ,