if ANTLR3's header files are not installed into the /usr/include, or
other directories searched by compiler by default. there are chances,
we cannot build the tree. so we have to find it first. as /opt/scylladb
is the directory where `scylla-antlr35-c++-dev` is installed on
debian derivatives, this directory is added so the find package module
can find the header files.
```
In file included from /home/kefu/dev/scylla/db/legacy_schema_migrator.cc:38:
In file included from /home/kefu/dev/scylla/cql3/util.hh:21:
/home/kefu/dev/scylla/build/cmake/cql3/CqlParser.hpp:55:10: fatal error: 'antlr3.hpp' file not found
^~~~~~~~~~~~
```
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
