libavcodec: Don't use dllexport, only dllimport when building DLLs

The only purpose of dllexport (which is set while building the library
that exports the symbols) is to have the linker automatically
export such symbols into a DLL without using a def file - it doesn't
affect the generated code.

For both MSVC and mingw builds, this isn't essential since we override
what symbols to export via an autogenerated def file instead.

Update a comment in configure to refer to the right concept.

With lld, this avoids warnings about duplicate export directives,
when some symbols are requested to be exported both via dllexport
attributes and via the autogenerated def file.

This also reduces the number of lines of code marginally.

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