win32: only use dllexport when building DLL

If building a static library, marking symbols as dllexport causes them
to be exported from the final executable. For example, run
objdump -x opus_demo.exe on a --disabled-shared build and look for the
export table; there should not be one in a normal Win32 .exe file, but
when linking static libopus, the exe exports all of the opus_* public
functions.

Use the libtool-defined DLL_EXPORT flag to determine whether we are
building a DLL and only specify __declspec(dllexport) in that case.