Fix MSVC installations with multiple "configure" files detected

Fix MSVC installations with multiple "configure" files detected

When installing binaries and libraries using the MSVC installation
routines, the operation gets done after moving to the root folder, whose
location is detected by checking if "configure" exists two times in a
row.  So, calling the installation script from src/tools/msvc/ with an
extra "configure" file four levels up the root path of the code tree
causes the execution to go further up, leading to a failure in finding
the builds.  This commit fixes the issue by moving to the root folder of
the code tree only once, when necessary.

Author: Arnold Müller
Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/16343-f638f67e7e52b86c@postgresql.org
Backpatch-through: 9.5