The message comes from make/common/shared/Platform.gmk:
  # How much RAM does this machine have:
  MB_OF_MEMORY := $(shell \
    if [ -f "C:/cygwin/bin/free.exe" ] ; then \
      ( C:/cygwin/bin/bash.exe -c "C:/cygwin/bin/free.exe -m" ) | \
grep Mem: | \
sed -e 's@\ \ *@ @g' | cut -d' ' -f2 ; \
    else \
      echo "512"; \
    fi)

The call to free.exe causes a message such as this on some windows machines
but not others:
   Unknown HZ value! (500) Assume 100.

The '500' varies depending on if the make is done under cygwin or MKS.