Now it's obvious: my problem (and solution) with FreeBSD and Adam's problem
with
debian are different; so FreeBSD is off-topc here.

Anyway I seem

 if (${CMAKE_SYSTEM_NAME} STREQUAL  "FreeBSD")
   set(TIDY_NAMES tidy5)

is a normal solution when an OS manages some different version of a library
renaming it.
In this specific case, it fixes incompatibility between headers and linked
library. Properly:

/usr/local/include/tidy/tidy.h and tidy.so are libtidy.0.99 (www/tidy-lib
port)

while the right is

/usr/local/include/tidy.h and tidy5.so (www/tidy-html5 port)

cmake finds the correct tidy.h but it cannot find the right ".so" without:
set(TIDY_NAMES tidy5), of course a better solution is welcome.

Alfonso