autogen.sh should check pkg-config's presence

I've stuck on building libxml2 for a couple of days by the following error message:

$ ./autogen.sh

<stripped...>

Checking zlib
./configure: line 13072: syntax error near unexpected token `Z,zlib,'
./configure: line 13072: `        PKG_CHECK_MODULES(Z,zlib,'

Now type 'make' to compile libxml2.
$ make
make: *** No targets specified and no makefile found.  Stop.

and later found out it is due to the missing pkg-config dependency, which is not documented in the F.A.Q.. IMO the autogen or the configure script should warn about this.