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.

include/opus_defines.h

@@ -65,7 +65,7 @@ extern "C" {
 
 #ifndef OPUS_EXPORT
 # if defined(WIN32)
-#  ifdef OPUS_BUILD
+#  if defined(OPUS_BUILD) && defined(DLL_EXPORT)
 #   define OPUS_EXPORT __declspec(dllexport)
 #  else
 #   define OPUS_EXPORT