[cairo] Pixman configure fail on mingw32

[Gerdus van Zyl]

Pixman fails to run make at the check for GTK on mingw32/msys; I do
not have the gtk dev libraries installed; commenting it out (adding
$enable_gtk = no) makes it compile fine. Seems like the this bug
(https://bugs.freedesktop.org/show_bug.cgi?id=4702) only now on
pixman.

./configure: line 12189: PKG_PROG_PKG_CONFIG: command not found
./configure: line 12191: syntax error near unexpected token `gtk+-2.0,'
./configure: line 12191: `   PKG_CHECK_EXISTS(gtk+-2.0,
enable_gtk=yes, enable_gtk=no)'
make: *** [config.status] Error 2

~Gerdus