[Bug 1391298] [NEW] MultiArchCross erroneously injects multiarch paths on compat < 9

[Mark Nunberg]

Public bug reported:

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.

** Affects: cmake (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1391298

Title:
  MultiArchCross erroneously injects multiarch paths on compat < 9

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