doc: Update configure section with popcnt details.
Popcnt instruction can be used to speedup hash computation on processors
with POPCNT support.
@@ -265,6 +265,12 @@ intrinsics can be used by passing ``-msse4.2``::
$ ./configure CFLAGS="-g -O2 -msse4.2"`
+Also builtin popcnt instruction can be used to speedup the counting of the
+bits set in an integer. For example on X86_64 with POPCNT support, it can be
+enabled by passing ``-mpopcnt``::
+
+ $ ./configure CFLAGS="-g -O2 -mpopcnt"`
+
If you are on a different processor and don't know what flags to choose, it is
recommended to use ``-march=native`` settings::
