Warnings during make

[Jean-Charles Malahieude]

On a fresh repo, as of cf3b8d901949a4a6d811755ec8d6afa2a1f5a067
the build is successful, but:

Making lily/out/main.o < cc
main.cc: In function 'void parse_argv(int, char**)':
main.cc:641:23: warning: loop variable 'keyval' creates a copy from type 
'const std::pair<std::__cxx11::basic_string<char>, 
std::__cxx11::basic_string<char> >' [-Wrange-loop-construct]
  641 |       for (const auto keyval : init_scheme_variables_global)
      |                       ^~~~~~
main.cc:641:23: note: use reference type to prevent copying
  641 |       for (const auto keyval : init_scheme_variables_global)
      |                       ^~~~~~
      |                       &

[...]

Making lily/out/program-option-scheme.o < cc
program-option-scheme.cc: In function 'scm_unused_struct* 
ly_command_line_options()':
program-option-scheme.cc:423:19: warning: loop variable 'keyval' creates 
a copy from type 'const std::pair<std::__cxx11::basic_string<char>, 
std::__cxx11::basic_string<char> >' [-Wrange-loop-construct]
  423 |   for (const auto keyval : init_scheme_variables_global)
      |                   ^~~~~~
program-option-scheme.cc:423:19: note: use reference type to prevent copying
  423 |   for (const auto keyval : init_scheme_variables_global)
      |                   ^~~~~~
      |                   &