Force to rerun cmake before building from the custom build target.

This is needed thanks to our awesome way to list files to add to a project (no comment please).

It also fixes a kind of bug, that caused certain project build twice. the problem was that we rely on the ${binary_dir } variable to know from where run the build command of each project but it was empty for DCMTK and the command was running by default from {CMAKE_BINARY_DIR}. in other words instead of build DCMTK we were just building one more time the whole super project...

I also take the oportunity to integrate the PR #33.

The PR#43 is also integrated, except the DDCMTK_OVERWRITE_WIN32_COMPILER_FLAGS:BOOL=OFF. (i can't tell wich is the best i just know that i had some trouble in other projects with this variable to ON)