misc: Create new glslang directory if needed

glslang has now moved to git. The update script will
recreate the glslang folder if the existing one is
the svn version.
diff --git a/update_external_sources.sh b/update_external_sources.sh
index 6ffaf5e..48c77fd 100755
--- a/update_external_sources.sh
+++ b/update_external_sources.sh
@@ -74,7 +74,7 @@
    make install
 }
 
-if [ ! -d "$BASEDIR/glslang" ]; then
+if [ ! -d "$BASEDIR/glslang" -o ! -d "$BASEDIR/glslang/.git" ]; then
    create_glslang
 fi
 if [ ! -d "$BASEDIR/LunarGLASS" ]; then