CMake: Use list(APPEND FOO) over set(FOO ${FOO} ...)

[ferdnyc]

set(FOO ${FOO} BAR BAZ)
set(FOO "${FOO};BAR;BAZ")

# They're both just funny ways to spell...
list(APPEND FOO BAR BAZ)

This PR replaces all remaining cargo-culted occurrences of set() being used to extend a list variable in CMake code, for which list(APPEND) is the appropriate command.

Not touched are calls like:

set(FOO_STRING "${FOO_STRING} extended-string-stuff")

...which are not list(APPEND) equivalent.

[kontura]

Thanks