configure: Stop using dlltool to create an import library

There shouldn't be any functional difference between the import
library created by dlltool and the one produced by the linker itself.
Keep installing it with both names though, for compatibility with users
that expect both to exist.

The dlltool step was added in ec10a9ab; prior to that, a MSVC
compatible import library was generated using lib.exe. After that
commit, there was no functional difference between the two import
libraries (and since MSVC 2012, link.exe works just fine with the
GNU binutils generated one).

Signed-off-by: Martin Storsjö <martin@martin.st>