configure: fix cross pkg-config detection

When cross-compiling, CURL_CHECK_PKGCONFIG was checking for the cross
pkg-config using ${host}-pkg-config.

The gold standard for doing this correctly is pkg-config's own macro,
PKG_PROG_PKG_CONFIG.  However, on the assumption that you have a good
reason not to use that directly (reduced dependencies for maintainer
builds?), the behaviour of cURL's version should at least match.
PKG_PROG_PKG_CONFIG uses AC_PATH_TOOL, which ultimately ends up trying
${host_alias}-pkg-config; this is not quite the same as what cURL does,
and may differ because ${host} has been run through config.sub.  For
instance, when cross-building to the armhf architecture on Ubuntu,
${host_alias} is arm-linux-gnueabihf while ${host} is
arm-unknown-linux-gnueabihf.  This may also have been the cause of the
problem reported at http://curl.haxx.se/mail/lib-2012-04/0224.html.

AC_PATH_TOOL is significantly simpler than cURL's current code, and
dates back to well before the current minimum of Autoconf 2.57, so let's
use it instead.