build-mingw-w64: Fix installing COPYING files on macOS

The install(1) tool on macOS doesn't support the -D option for
creating directories at the same time while installing files.
Therefore, run a separate step for creating directories. Doing that
with "mkdir -p", even if install(1) also could do that with the -d
option (likewise with a separate command).

Also stop specifying the destination file name, when it's equal to
the source files, now that we know that the destination directory
already exists.

build-mingw-w64-libraries.sh

@@ -47,7 +47,8 @@ for lib in winpthreads winstorecompat; do
         make -j$CORES
         make install
         cd ..
-        install -Dm644 COPYING "$arch_prefix/share/mingw32/COPYING.${lib}.txt"
+        mkdir -p "$arch_prefix/share/mingw32"
+        install -m644 COPYING "$arch_prefix/share/mingw32/COPYING.${lib}.txt"
     done
     cd ..
 done