build-compiler-rt: Only use a temp directory as install prefix if necessary

This makes the build script output more legible, when the cmake
install output actually shows where the files are installed in the
end.

build-compiler-rt.sh

@@ -81,8 +81,16 @@ else
 fi
 
 cd llvm-project/compiler-rt
-# Use a staging directory in case parts of the resource dir are immutable
-WORKDIR=$(mktemp -d); trap "rm -rf $WORKDIR" 0
+
+INSTALL_PREFIX="$CLANG_RESOURCE_DIR"
+
+if [ -h "$CLANG_RESOURCE_DIR/include" ]; then
+    # Symlink to system headers; use a staging directory in case parts
+    # of the resource dir are immutable
+    WORKDIR="$(mktemp -d)"; trap "rm -rf $WORKDIR" 0
+    INSTALL_PREFIX="$WORKDIR/install"
+fi
+
 
 for arch in $ARCHS; do
     if [ -n "$SANITIZERS" ]; then
@@ -125,23 +133,24 @@ for arch in $ARCHS; do
         -DCMAKE_CXX_FLAGS_INIT="$CFGUARD_CFLAGS" \
         $SRC_DIR
     cmake --build . ${CORES:+-j${CORES}}
-    cmake --install . --prefix "${WORKDIR}/install"
+    cmake --install . --prefix "$INSTALL_PREFIX"
     mkdir -p "$PREFIX/$arch-w64-mingw32/bin"
     if [ -n "$SANITIZERS" ]; then
-        mv "${WORKDIR}/install/lib/windows/"*.dll "$PREFIX/$arch-w64-mingw32/bin"
+        mv "$INSTALL_PREFIX/lib/windows/"*.dll "$PREFIX/$arch-w64-mingw32/bin"
     fi
     INSTALLED=1
     cd ..
 done
 
-if [ -z "$INSTALLED" ]; then
-    # Don't try to move the installed files in place, if nothing was
-    # installed (e.g. if building with --build-sanitizers but not for x86).
-    exit 0
-fi
+if [ "$INSTALL_PREFIX" != "$CLANG_RESOURCE_DIR" ]; then
+    if [ -z "$INSTALLED" ]; then
+        # Don't try to move the installed files in place, if nothing was
+        # installed (e.g. if building with --build-sanitizers but not for x86).
+        exit 0
+    fi
 
-if [ -h "$CLANG_RESOURCE_DIR/include" ]; then
     # symlink to system headers - skip copy
-    rm -rf ${WORKDIR}/install/include
+    rm -rf "$INSTALL_PREFIX/include"
+
+    cp -r "$INSTALL_PREFIX/." $CLANG_RESOURCE_DIR
 fi
-cp -r ${WORKDIR}/install/. $CLANG_RESOURCE_DIR