mingw: Use Makefile.unix to build opusfile.

This is faster. We still don't have good dependency tracking
among the components, but roughly:

    make
    make opusfile-0.dll
    make package

Should automate the basics of putting together a win32 binary
package. It still doesn't include documentation. See the
mingw/README for additional steps there.

Also, don't copy dll.a files. They don't seem to be necessary.

mingw/Makefile

@@ -99,14 +99,12 @@ $(DIST): $(addprefix $(CURDIR)/bin/, libogg-0.dll libopus-0.dll)
 	#cd .. && make install
 	mkdir -p $(DIST)
 	cp ../AUTHORS ../COPYING ../README.md ../include/opusfile.h $@
-	cp ../.libs/libopusfile-0.dll $@
-	cp ../.libs/libopusfile.a $@
-	cp ../.libs/libopusfile.dll.a $@
-	cp ../.libs/libopusurl-0.dll $@
-	cp ../.libs/libopusurl.a $@
-	cp ../.libs/libopusurl.dll.a $@
+	cp .libs/libopusfile-0.dll $@
+	cp .libs/libopusfile.a $@
+	cp .libs/libopusurl-0.dll $@
+	cp .libs/libopusurl.a $@
 	cp bin/*.dll $@
-	cp ../examples/.libs/*.exe $@
+	cp *.exe $@
 	cp /usr/i686-w64-mingw32/sys-root/mingw/bin/libgcc_s_dw2-1.dll $@
 	cp /usr/i686-w64-mingw32/sys-root/mingw/bin/libwinpthread-1.dll $@
 	i686-w64-mingw32-strip $@/*.exe

mingw/README.md

@@ -1,6 +1,6 @@
 # Cross-compiling under mingw
 
-Just running `make` in this directory should download
+Just running `make libopusfile-0.dll` in this directory should download
 and build opusfile and its dependencies. Some mingw
 libraries need to be compiled into the final package.
 
@@ -44,12 +44,13 @@ The steps are something like
 - Convert README.md to DOS line endings.
 - Copy .libs/libopusfile-0.dll to the release dir.
 - Copy .libs/libopusfile.a to the release dir.
-- Copy .libs/libopusfile.dll.a to the release dir. (May not be needed?)
 - Copy .libs/libopusurl-0.dll to the release dir.
 - Copy .libs/libopusurl.a to the release dir.
-- Copy .libs/libopusurl.dll.a to the release dir. (May not be needed?)
 - Copy mingw/bin/*.dll to the release dir for dependencies.
-- Copy any other dependent dlls, e.g. on Fedora 23 I needed to copy
+- Copy any other dependent dlls, e.g. on Fedora 32 I needed to copy
+    /usr/i686-w64-mingw32/sys-root/mingw/bin/libgcc_s_dw2-1.dll
+    /usr/i686-w64-mingw32/sys-root/mingw/bin/libwinpthread-1.dll
+  On Fedora 23 I needed to copy
     /usr/i686-w64-mingw32/sys-root/mingw/bin/libgcc_s_sjlj-1.dll
     /usr/i686-w64-mingw32/sys-root/mingw/bin/libwinpthread-1.dll
   On Gentoo I needed to copy