[CMake] Make CMake the default buildsystem.
[CMake] Check for $CMAKE outside opts file.

CMAKE is nom handled similar to MAKE.
CMAKE is no longer evaluated in the opts file as this would overwrite
a CMAKE set by the dunecontrol call.
This fixes FS#1518.
[CMake] Default to use the build-dir.

The only case we do not want to use the build-dir is
when explicitly autotools are used.
[dunecontrol] Accumulate checks for environment variables.
[dunecontrol][CMake] Reintroduce evaluation of CMAKE from opts file.

CMAKE can now be set in the shell or the opts file, if both are set
the shell has higher priority.
This feature was dropped in …aef001c21e3a9 to fix FS#1518.
[dunecontrol] Warn less pushy that build directory is not set.

The warning was spilled out for every module for every command,
even for update.
The build directory is announced by CMake's configure anyway.
Now it is printed for make, too.
[buildsystem] By default CMake is used instead of autotools.

Pass --no-cmake to dunecontrol to force the use of autotools.
If no build directory is given, make files, libararies and
executables are in <module-dir>/build-cmake/ .
Further information in FS#1000 and FS#1500.