Matplotlib: make it discover pkg-config files

Fixes #45. This has been fixed in hashstack1, the issue is that the only way to
force Matplotlib to use our png and our freetype (as opposed to systewide) is
by using pkg-config. So we just need to let it know where the .pc files are.
Add and use libflags
updated libpng url to what we hope is a permanent one
Merge pull request #48 from certik/mpl2

Mpl fix rpath