build system: use consistently pkg-config to check for dependencies

Rather than rely on the old m4 macros for libxml2 and libcurl, which were
also misused, always use PKG_CHECK_MODULES (also assuming a new enough
version that calls AC_SUBST by itself), and fix the Makefile.am to use
_CFLAGS properly as well.

Signed-off-by: Diego Elio 'Flameeyes' Pettenò <flameeyes@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>