Quote uses of $PREFIX in the build script, to improve support for installing into a path that contains spaces

For build-mingw-w64.sh to succeed when installing to such a path,
the mingw-w64 version has to be updated to the latest git master.

build-mingw-w64-libraries.sh still fails in this case, as libtool
fails to properly quote $LD in a test for whether the linker is GNU ld.

build-mingw-w64-libraries.sh

@@ -22,7 +22,7 @@ for lib in winpthreads winstorecompat; do
     for arch in $ARCHS; do
         mkdir -p build-$arch
         cd build-$arch
-        ../configure --host=$arch-w64-mingw32 --prefix=$PREFIX/$arch-w64-mingw32 --libdir=$PREFIX/$arch-w64-mingw32/lib
+        ../configure --host=$arch-w64-mingw32 --prefix="$PREFIX/$arch-w64-mingw32" --libdir="$PREFIX/$arch-w64-mingw32/lib"
         make -j$CORES
         make install
         cd ..