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.

wrappers/clang-target-wrapper.sh

@@ -71,4 +71,4 @@ FLAGS="$FLAGS -fuse-ld=lld"
 FLAGS="$FLAGS -fuse-cxa-atexit"
 FLAGS="$FLAGS -Qunused-arguments"
 
-$CCACHE $CLANG $FLAGS "$@"
+$CCACHE "$CLANG" $FLAGS "$@"