squid: fix static build with libxml2

Use pkg-config to find libxml2 to fix static build of squid
Add SQUID_AUTORECONF = YES and remove ac_cv_libxml2_include (not needed
anymore)

Fixes:
 - http://autobuild.buildroot.org/results/7f23eb98c311b294c7f0e165279fa26909a5ff93



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>