aom/aom_encoder.h: remove configure option reference

and update it to the CMake equivalent

Bug: aomedia:2673
Change-Id: Ied3e295cd1ffe93500b21e3937eaafebf1438e0f
(cherry picked from commit a786b2d355e414a5bb64ed201403fc961f50d331)

aom/aom_encoder.h

diff --git a/aom/aom_encoder.h b/aom/aom_encoder.h
index a0efd51..a629eef 100644
--- a/aom/aom_encoder.h
+++ b/aom/aom_encoder.h
@@ -904,7 +904,7 @@
  * function directly, to ensure that the ABI version number parameter
  * is properly initialized.
  *
- * If the library was configured with --disable-multithread, this call
+ * If the library was configured with -DCONFIG_MULTITHREAD=0, this call
  * is not thread safe and should be guarded with a lock if being used
  * in a multithreaded context.
  *