[CMake] out of source command line flag

[Alexander Neundorf]

On Saturday 05 September 2009, Eric Noulard wrote:
> Continued....
...
> CMake may guess which one is the build tree using this algorithm.
>
> 1) If there is a CMakeCache.txt in the dir then this is a build tree
> 2) If the path does not exists then this is a "to be created" build tree
>
> error cases are:
>
> E1) One gives two existing dir with none containing a CMakeCache.txt
>       ==> send an error message explaining that and abort.
>
> E2) One gives two directories
>           E21) both contains a CMakeCache.txt
>       or E22) one contains a CMakeCache.txt and the other does not exists.
>
>           E21 is plain wrong
>           E22 the source tree may have been polluted with a previous
> in-source config.
>
> E3) One gives a build tree which does not correspond to the given source
> tree This is easy to check because CMakeCache.txt contains
>                  <PROJECT>_SOURCE_DIR
>
>       If this appear CMake should abort and send a message telling
>       the source tree/build tree mismatch.

I think it could be easier than that.
E1) none of the two directories contains a CMakeLists.txt 
E2) both directories contain a CMakeLists.txt 

In all other case, one directory contains a CMakeLists.txt, this is the source 
dir. The other directory does not contain a CMakeLists.txt, so it is not a 
source dir. It may contain a CMakeCache.txt, then it is already a build dir. 
If it does not, it will become the build dir.

Alex