Support cross compilation to non-windows targets

Don't have --host/$HOST imply targeting Windows, but actually inspect
the target triple where necessary.

build-lldb-mi.sh

@@ -89,10 +89,21 @@ fi
 if [ -n "$HOST" ]; then
     BUILDDIR=$BUILDDIR-$HOST
 
-    CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_SYSTEM_NAME=Windows"
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_C_COMPILER=$HOST-gcc"
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_CXX_COMPILER=$HOST-g++"
-    CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_RC_COMPILER=$HOST-windres"
+    case $HOST in
+    *-mingw32)
+        CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_SYSTEM_NAME=Windows"
+        CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_RC_COMPILER=$HOST-windres"
+        ;;
+    *-linux*)
+        CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_SYSTEM_NAME=Linux"
+        ;;
+    *)
+        echo "Unrecognized host $HOST"
+        exit 1
+        ;;
+    esac
 
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_FIND_ROOT_PATH=$LLVM_DIR"
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_FIND_ROOT_PATH_MODE_PROGRAM=NEVER"