Macroprocessor: under Windows, trim trailing slashes and backslashes in the...

Macroprocessor: under Windows, trim trailing slashes and backslashes in the value of “@#includepath”

This is a  workaround for a GCC/MinGW bug present in version 10.2
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88881), that affects
gcc-mingw-w64 in Debian “Bullseye” 11. It is fixed in GCC 10.3, and thus should
be fixed in Debian “Bookworm” 12.

Ref. Madysson/estimation-codes#11