MultiArchCross erroneously injects multiarch paths on compat < 9

[Mark Nunberg]

My project's `compat` file is set to `7`, and is thus multi-arch unaware; however when using CMake on Trusty, it seems to forcefully use the multi-arch installation prefix, by means of the `GNUInstallDirs.cmake` (a standard CMake module). This actually took me quite a while to debug since this behavior only manifested itself when running under a debian build environment (i.e. the various DEB_* environment variables set), otherwise I don't see it.

Now, because GNUInstallDirs will try to use a multi-arch install prefix by default, what my CMakeLists.txt does is have something along the lines of:

```
IF (NOT CMAKE_INSTALL_LIBDIR)
  SET(LIBDIR "lib")
ENDIF()
```

However, this yet still insufficient, because in addition to injecting MultiArchCross into GNUInstallDirs, it also injects itself into CMakeDetermineCompiler, basically ensuring that unless this directive is defined explicitly on the commandline, CMAKE_INSTALL_LIBDIR will always be set, and that little check over there will fail.

[Gianfranco Costamagna (costamagnagianfranco)]

this is true, however I'm not aware of a trivial fix to check this :(

[Launchpad Janitor (janitor)]

[Expired for cmake (Ubuntu) because there has been no activity for 60 days.]