Unescaped variables in pkgconfig files

Spaces in variable values are escaped with \, this is required in the case the value is a path that and is used in cflags or libs arguments. This was an undocumented behaviour that caused issues in the case the variable is a space separated list of items.

For backward compatibility reasons this behaviour could not be changed, new keyword arguments have thus been added: unescaped_variables and unescaped_uninstalled_variables.

pkg = import('pkgconfig')
...
pkg.generate(lib,
  variables: {
    'mypath': '/path/with spaces/are/escaped',
  },
  unescaped_variables: {
    'mylist': 'Hello World Is Not Escaped',
  },
)