OpenCV ports

[Hiroo Ono (小野寛生)]

Hello,

I am trying to update OpenCV port from 3.4.1 to 3.4.5.
and another person is making 4.0.1 port.
https://forums.freebsd.org/threads/make-extract-extracts-two-files-to-the-same-location.69940/

Regarding PR 234147, I am trying to make OpenCV 3.4.5 and 4.0.1 can be installed
at the same time.
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=234147

To be able to install both versions at the same time, I set
-DINSTALL_TO_MANGLED_PATH:BOOL=ON
option of OpenCV cmake. The install paths are as below:
    /usr/local/bin/opencv-3.4.5
    /usr/local/lib/opencv-3.4.5
    /usr/local/share/OpenCV/3.4.5

I submitted an initial patch here, but there are some problems.
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=237135

problem 1: for the python library, I did not know how to make two
version of a library coexists. Is there any solution?

problem 2: INSTALL_TO_MANGLED_PATH option make directories which
contain full version in their name.
Ports dependent on opencv port have to make change in their Makefile every time
opencv3 port is updated (not so often, I think).
Is this acceptable? If no, what directory layout should I choose?
Note that to make directory layout different, a little surgery on
cmakefiles is necessary.