configure: Strip ordinals from mingw generated def files

configure: Strip ordinals from mingw generated def files

The def files are used for generating import libraries for
other toolchains (in particular, for generating import libraries
for MSVC for DLLs built with mingw).

The def files produced by mingw/gcc contains ordinals for each
exported function. When MSVC tools generate import libraries
from such a def file, MSVC links to the DLL by the ordinals
instead of linking by name.

Since the def files aren't maintained by hand, the ordinal
numbers are assigned (more or less) randomly and any caller
linking to the libs by ordinals will break as soon as the libraries
export more/fewer functions.

Therefore, strip out the ordinals from the generated def files,
to make users link to the libraries by name.

Callers linking to the DLLs using the gcc provided import library
link by name as they should.

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