Our pkg-config wrapper relies on the ability to redefine the $prefix and
$exec_prefix variables in order to construct proper search paths relative
to the build environment.

Patch the .pc file template to construct libdir, sharedlibdir and includedir
relative to the ${prefix} variable so that it can be overridden as needed.

This also fixes the libxml2/host build issue raised at
https://github.com/openwrt/packages/issues/6073 - it was caused by libxml2's
configure picking up a wrong host search path through zlib.pc, letting it
include the wrong endian.h, causing spurious member redeclaration errors in
system headers.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>