forced reinstallation of ROOT fails with missing vdt/vdtMath.h

[boegel]

Forced reinstalled of (for example) ROOT-6.14.06-foss-2018b-Python-2.7.15.eb is failing during make install with:

In file included from input_line_12:7:
/tmp/kehoste/easybuild/build/ROOT/6.14.06/foss-2018b-Python-2.7.15/easybuild_obj/include/ROOT/RVec.hxx:36:10: fatal error: 'vdt/vdtMath.h' file not found                                                                                                                     
#include <vdt/vdtMath.h>
         ^~~~~~~~~~~~~~~                                                                                                               
Error: /tmp/kehoste/easybuild/build/ROOT/6.14.06/foss-2018b-Python-2.7.15/easybuild_obj/bin/rootcling: compilation failure (/tmp/kehoste/easybuild/build/ROOT/6.14.06/foss-2018b-Python-2.7.15/easybuild_obj/lib/libROOTVecOpsc24bc4f618_dictUmbrella.h)                      
make[2]: *** [math/vecops/G__ROOTVecOps.cxx] Error 1                                                                                   
make[2]: Leaving directory `/tmp/kehoste/easybuild/build/ROOT/6.14.06/foss-2018b-Python-2.7.15/easybuild_obj'                          
make[1]: *** [math/vecops/CMakeFiles/G__ROOTVecOps.dir/all] Error 2                                                                    
make[1]: Leaving directory `/tmp/kehoste/easybuild/build/ROOT/6.14.06/foss-2018b-Python-2.7.15/easybuild_obj'                          
make: *** [all] Error 2

This occurs because vdt/vdtMath.h was found in the existing ROOT installation directory at configure time, but since the installation directory is removed right before make install, the file is no longer there when make install tries to access it during the installation step.