[cairo] another (win32) bootstrap problem

[Jason Dorje Short]

When I run

   ./configure --disable-freetype --disable-xlib

I get the following error in configure:

checking for zlib.h... yes
./configure: line 23881: --exists: command not found
./configure: line 23881: --exists: command not found
./configure: line 23881: --exists: command not found
configure: WARNING: Could not find libpng in the pkg-config search path
checking for GLITZ_CFLAGS...
checking for GLITZ_LIBS...
checking for PIXMAN_CFLAGS...
checking for PIXMAN_LIBS...
configure: error: The pkg-config script could not be found or is too 
old.  Make sure it
is in your PATH or set the PKG_CONFIG environment variable to the full
path to pkg-config.

The problem is that the PNG backend checks use $PKG_CONFIG (the 
environment variable, that is), which is not set up inside configure 
(it's set up inside PKG_CHECK_MODULES but this isn't called if freetyle 
and xlib are disabled).

-jason