build-llvm: Support using --with-clang in a --host cross build

When cross compiling with llvm-mingw, we already implicitly use
clang and lld, but we don't need to specify these options as they
are set implicitly via the llvm-mingw wrappers.

build-llvm.sh

@@ -157,9 +157,18 @@ CMAKEFLAGS="$LLVM_CMAKEFLAGS"
 
 if [ -n "$HOST" ]; then
     ARCH="${HOST%%-*}"
-    CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_C_COMPILER=$HOST-gcc"
-    CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_CXX_COMPILER=$HOST-g++"
-    CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_SYSTEM_PROCESSOR=$ARCH"
+
+    if [ -n "$WITH_CLANG" ]; then
+        CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_C_COMPILER=clang"
+        CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_CXX_COMPILER=clang++"
+        CMAKEFLAGS="$CMAKEFLAGS -DLLVM_USE_LINKER=lld"
+        CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_C_COMPILER_TARGET=$HOST"
+        CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_CXX_COMPILER_TARGET=$HOST"
+    else
+        CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_C_COMPILER=$HOST-gcc"
+        CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_CXX_COMPILER=$HOST-g++"
+        CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_SYSTEM_PROCESSOR=$ARCH"
+    fi
     case $HOST in
     *-mingw32)
         CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_SYSTEM_NAME=Windows"