ninja: Fix multiline arguments

[xclaesse]

When using a custom_target() and an argument contains a newline, the command gets wrapped and written into a pickle file. However, when it's a compiler argument, the ninja backend was raising an error.

[xclaesse]

Hmm, on 2nd though, I'm not sure this is right. It works because C preprocessor will handle "hello\nworld" to insert a newline character, but it's not the same thing as passing an actual newline char to command's argv.