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