Fix `make install` on Windows

Since `Makefile` passes `--destdir="$$(DESTDIR)/"` to `install.sh`,
`make install` tries to install libraries to
`$CFG_DESTDIR$CFG_PREFIX/$CFG_LIBDIR_RELATIVE` which is usually
`//path/to/usr/$CFG_LIBDIR_RELATIVE`.

The POSIX spec [1] states that if path begins with `//` it is
implementation-defined.
Usual systems treat them as normal abaolute path, but cygwin and MSYS
does not! They use `//hostname/path` syntax for network drives.
This caused `make install` issue on Windows.

This patch removes `/` of destdir to solve the issue.

[1]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_12