CMake: Reverts usage of `string(APPEND ...)`.
CMake: Reverts usage of `string(APPEND ...)`.

This partially reverts commit 4ce689c8e2be386d0086acce63ce5b8bbab6c34b.

`string(APPEND ...)` was added in CMake 3.4, but the earliest required
version of CMake is 2.8.12.  `list(APPEND ...)` exists in 2.8.12,
so that pattern will not be reverted.

andschwa/mesos,neilconway/mesos,abudnik/mesos,reneploetz/mesos,jpeach/mesos,chhsia0/mesos,gsantovena/mesos,jpeach/mesos,neilconway/mesos,gsantovena/mesos,dforsyth/mesos,reneploetz/mesos,chhsia0/mesos,kaysoky/mesos,zmalik/mesos,Gilbert88/mesos,gsantovena/mesos,dforsyth/mesos,asamerh4/mesos,dforsyth/mesos,gsantovena/mesos,chhsia0/mesos,andschwa/mesos,Gilbert88/mesos,verizonlabs/mesos,jpeach/mesos,zmalik/mesos,Gilbert88/mesos,neilconway/mesos,abudnik/mesos,kaysoky/mesos,andschwa/mesos,reneploetz/mesos,neilconway/mesos,verizonlabs/mesos,dforsyth/mesos,verizonlabs/mesos,asamerh4/mesos,asamerh4/mesos,abudnik/mesos,abudnik/mesos,zmalik/mesos,andschwa/mesos,dforsyth/mesos,kaysoky/mesos,neilconway/mesos,jpeach/mesos,asamerh4/mesos,chhsia0/mesos,Gilbert88/mesos,gsantovena/mesos,asamerh4/mesos,reneploetz/mesos,andschwa/mesos,neilconway/mesos,verizonlabs/mesos,chhsia0/mesos,reneploetz/mesos,gsantovena/mesos,dforsyth/mesos,zmalik/mesos,chhsia0/mesos,asamerh4/mesos,abudnik/mesos,andschwa/mesos,zmalik/mesos,verizonlabs/mesos,Gilbert88/mesos,jpeach/mesos,chhsia0/mesos,kaysoky/mesos,zmalik/mesos,gsantovena/mesos,verizonlabs/mesos,abudnik/mesos,jpeach/mesos,asamerh4/mesos,reneploetz/mesos,kaysoky/mesos,dforsyth/mesos,reneploetz/mesos,abudnik/mesos,Gilbert88/mesos,Gilbert88/mesos,jpeach/mesos,zmalik/mesos,neilconway/mesos,andschwa/mesos,kaysoky/mesos,verizonlabs/mesos,kaysoky/mesos