bin/install_megadrivers.py: Correctly handle DESTDIR=''

Currently if destdir is set to '' then the resulting libdir will have
it's first character replaced by / instead of / being prepended to the
string. This was the result of ensuring that that DESTDIR wouldn't be
ignored if libdir was absolute, since the only cases that meson allows
the libdir to be absolute is if the prefix is /, this won't be a
problem.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110211


Fixes: ae3f45c1
       ("bin/install_megadrivers: fix DESTDIR and -D*-path")
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit 4188dd78)