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-compiler-rt.sh

@@ -29,7 +29,7 @@ export PATH="$PREFIX/bin:$PATH"
 : ${CORES:=4}
 : ${ARCHS:=${TOOLCHAIN_ARCHS-i686 x86_64 armv7 aarch64}}
 
-CLANG_VERSION=$(basename $(dirname $(dirname $(dirname $($PREFIX/bin/clang --print-libgcc-file-name -rtlib=compiler-rt)))))
+CLANG_VERSION=$(basename "$(dirname "$(dirname "$(dirname "$("$PREFIX/bin/clang" --print-libgcc-file-name -rtlib=compiler-rt)")")")")
 
 if [ ! -d llvm-project/compiler-rt ] || [ -n "$SYNC" ]; then
     CHECKOUT_ONLY=1 ./build-llvm.sh
@@ -38,8 +38,8 @@ fi
 # Add a symlink for i386 -> i686; we normally name the toolchain
 # i686-w64-mingw32, but due to the compiler-rt cmake peculiarities, we
 # need to refer to it as i386 at this stage.
-if [ ! -e $PREFIX/i386-w64-mingw32 ]; then
-    ln -sfn i686-w64-mingw32 $PREFIX/i386-w64-mingw32 || true
+if [ ! -e "$PREFIX/i386-w64-mingw32" ]; then
+    ln -sfn i686-w64-mingw32 "$PREFIX/i386-w64-mingw32" || true
 fi
 
 cd llvm-project/compiler-rt
@@ -80,12 +80,12 @@ for arch in $ARCHS; do
     cmake \
         ${CMAKE_GENERATOR+-G} "$CMAKE_GENERATOR" \
         -DCMAKE_BUILD_TYPE=Release \
-        -DCMAKE_INSTALL_PREFIX=$PREFIX/$arch-w64-mingw32 \
+        -DCMAKE_INSTALL_PREFIX="$PREFIX/$arch-w64-mingw32" \
         -DCMAKE_C_COMPILER=$arch-w64-mingw32-clang \
         -DCMAKE_CXX_COMPILER=$arch-w64-mingw32-clang++ \
         -DCMAKE_SYSTEM_NAME=Windows \
-        -DCMAKE_AR=$PREFIX/bin/llvm-ar \
-        -DCMAKE_RANLIB=$PREFIX/bin/llvm-ranlib \
+        -DCMAKE_AR="$PREFIX/bin/llvm-ar" \
+        -DCMAKE_RANLIB="$PREFIX/bin/llvm-ranlib" \
         -DCMAKE_C_COMPILER_WORKS=1 \
         -DCMAKE_CXX_COMPILER_WORKS=1 \
         -DCMAKE_C_COMPILER_TARGET=$buildarchname-windows-gnu \
@@ -93,14 +93,14 @@ for arch in $ARCHS; do
         -DCOMPILER_RT_USE_BUILTINS_LIBRARY=TRUE \
         $SRC_DIR
     make -j$CORES
-    mkdir -p $PREFIX/lib/clang/$CLANG_VERSION/lib/windows
-    mkdir -p $PREFIX/$arch-w64-mingw32/bin
+    mkdir -p "$PREFIX/lib/clang/$CLANG_VERSION/lib/windows"
+    mkdir -p "$PREFIX/$arch-w64-mingw32/bin"
     for i in lib/windows/libclang_rt.*-$buildarchname*.a; do
-        cp $i $PREFIX/lib/clang/$CLANG_VERSION/lib/windows/$(basename $i | sed s/$buildarchname/$libarchname/)
+        cp $i "$PREFIX/lib/clang/$CLANG_VERSION/lib/windows/$(basename $i | sed s/$buildarchname/$libarchname/)"
     done
     for i in lib/windows/libclang_rt.*-$buildarchname*.dll; do
         if [ -f $i ]; then
-            cp $i $PREFIX/$arch-w64-mingw32/bin
+            cp $i "$PREFIX/$arch-w64-mingw32/bin"
         fi
     done
     if [ -n "$SANITIZERS" ]; then