Avoid recompiling option_parser every time cmake runs.

Switch to `configure_file`, which won't touch the output file unless
the contents change.

cmake/FileToString.cmake

@@ -3,11 +3,13 @@
 # Create a C++ file `file_out` that defines a string named `string_name` in
 # `namespace`, which contains the contents of `file_in`.
 
+# Cache this so we can access it from wherever file_to_string is called.
+set(_nvbench_file_to_string_path "${CMAKE_CURRENT_LIST_DIR}/FileToString.in")
 function(file_to_string file_in file_out namespace string_name)
   file(READ "${file_in}" file_in_contents)
 
   set(file_out_contents)
-  string(APPEND file_out_contents
+  string(APPEND file_to_string_payload
     "#include <string>\n"
     "namespace ${namespace} {\n"
     "const std::string ${string_name} =\n"
@@ -15,5 +17,5 @@ function(file_to_string file_in file_out namespace string_name)
     "}\n"
   )
 
-  file(WRITE "${file_out}" "${file_out_contents}")
+  configure_file("${_nvbench_file_to_string_path}" "${file_out}")
 endfunction()

cmake/FileToString.in

@@ -0,0 +1 @@
+${file_to_string_payload}