py/mpconfig: Actually allow to override MICROPY_BYTES_PER_GC_BLOCK.
diff --git a/py/mpconfig.h b/py/mpconfig.h
index 922f865..ef078d3 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -98,7 +98,9 @@
// Number of bytes in memory allocation/GC block. Any size allocated will be
// rounded up to be multiples of this.
+#ifndef MICROPY_BYTES_PER_GC_BLOCK
#define MICROPY_BYTES_PER_GC_BLOCK (4 * BYTES_PER_WORD)
+#endif
// Number of words allocated (in BSS) to the GC stack (minimum is 1)
#ifndef MICROPY_ALLOC_GC_STACK_SIZE