In mingw builds, make our own import library for libperl.

In mingw builds, make our own import library for libperl.

Borrow the method already used by plpython.  This is pretty ugly, but
it might fix the build failure exhibited by buildfarm member narwhal
since commit 846e91e0223cf9f2821c3ad4dfffffbb929cb027.

Hiroshi Inoue