When on win32, it makes a difference, whether '...' or "..." is used in shell,
consider:

  C:\>pkg-config --print-errors 'cairo >= 1.4.12'
  Package 'cairo was not found in the pkg-config search path.
  Perhaps you should add the directory containing `'cairo.pc'
  to the PKG_CONFIG_PATH environment variable
  No package ''cairo' found

v.s.

  C:\>pkg-config --print-errors "cairo >= 1.4.12"
  C:\>pkg-config --print-errors "cairo >= 1.8.12"
  Requested 'cairo >= 1.8.12' but version of cairo is 1.6.4


So it is important to use double quotes.


/patch attached/