build-mingw-w64: Use make install-strip instead of building with -Wl,-s

These days, we have a working strip tool, so we can do a slightly
more straightforward procedure here.

In build-make.sh we still build with -Wl,-s, as we call a less
obvious install target there, in order to only install mingw32-make.sh,
not some header files and documentation.

build-mingw-w64.sh

@@ -143,9 +143,9 @@ cd mingw-w64-tools/widl
 for arch in $ARCHS; do
     mkdir -p build-$CROSS_NAME$arch
     cd build-$CROSS_NAME$arch
-    ../configure --prefix="$PREFIX" --target=$arch-w64-mingw32 $CONFIGFLAGS LDFLAGS="-Wl,-s"
+    ../configure --prefix="$PREFIX" --target=$arch-w64-mingw32 $CONFIGFLAGS
     make -j$CORES
-    make install
+    make install-strip
     cd ..
 done
 cd "$PREFIX/bin"