Fix the issue tigervnc-server and -viewer cannot be installed together

when DOCS=on because both ports install LICENSE_FILE into the same place.
Also, following changes are made.

net/tigervnc-{server,viewer}:

• bump PORTREVISION due to plist & dependency change
• do build tests, it introduces unnecessary dependencies on fltk

net/tigervnc-server:

• add missing dependency on devel/m4
• remove unnecessary dependency on x11-toolkits/fltk

MFH: 2019Q2