fix: Disable building of Colvars CUDA files in VMD

VMD's configure script is weird: it has a ton of variables to hold for CUDA source files for
various components, but only "$cuda_cu" is actually defined and used.

Author: giacomofiorin

devel-tools/generate-vmd-makefile.sh

@@ -3,7 +3,7 @@
 # Generate the file colvars_files.pl used by VMD (i.e. a poor person's version of a dependency file)
 
 if [ ! -f "${1}" ] ; then
-    echo "Usage: ${0} [... source files ...] [... header files ...] [... CUDA source files ...]" >& 2
+    echo "Usage: ${0} [... source files ...] [... header files ...]" >& 2
     exit 1
 fi
 
@@ -18,10 +18,11 @@ for file in "${files[@]}" ; do
     elif [ "${file%.h}" != "${file}" ] ; then
         header_files+=("${file}")
     elif [ "${file%.cu}" != "${file}" ] ; then
-        cuda_files+=("${file}")
+        echo "Error: building of Colvars CUDA files with VMD is not supported yet" >&2
+        # cuda_files+=("${file}")
     else
-       echo "Error: file ${file} has an unsupported extennsion" >&2
-       exit 1
+        echo "Error: file ${file} has an unsupported extension" >&2
+        exit 1
     fi
 done