loci: disable GCC warning unused-but-set-variable

It's more complicated to generate code that doesn't trigger this warning when
assertions are disabled.
diff --git a/c_gen/c_code_gen.py b/c_gen/c_code_gen.py
index 52d30f2..4aec103 100644
--- a/c_gen/c_code_gen.py
+++ b/c_gen/c_code_gen.py
@@ -757,6 +757,7 @@
  ****************************************************************/
 
 #pragma GCC optimize ("s")
+#pragma GCC diagnostic ignored "-Wunused-but-set-variable"
 
 #include <loci/loci.h>
 #include <loci/of_object.h>