[build] Add variable telling if DESTINATION is needed with install()

It seems DESTINATION was only allowed to be omitted in 3.14

https://cmake.org/cmake/help/v3.14/command/install.html#targets
vs
https://cmake.org/cmake/help/v3.13/command/install.html#targets

After that the install destination directories can be deduced.

CMakeLists.txt

@@ -43,6 +43,7 @@ set_if(ANDROID     ${SYSNAME_LC} MATCHES "android")
 set_if(SUNOS       "${SYSNAME_LC}" MATCHES "sunos")
 set_if(POSIX       LINUX OR DARWIN OR BSD OR SUNOS OR ANDROID OR (CYGWIN AND CYGWIN_USE_POSIX))
 set_if(SYMLINKABLE LINUX OR DARWIN OR BSD OR SUNOS OR CYGWIN OR GNU)
+set_if(NEED_DESTINATION  ${CMAKE_VERSION} VERSION_LESS "3.14.0")
 
 # Not sure what to do in case of compiling by MSVC.
 # This will make installdir in C:\Program Files\SRT then