Use AC_PATH_TOOL to find libgcrypt-config and xml2-config

Using AC_PATH_TOOL prefers ${CHOST}-prefixed tools over 'bare' ones,
therefore improving support for cross-compilation and multilib. This
is useful for environment where libgcrypt-config and xml2-config
correspond to the build host environment, while
${CHOST}-libgcrypt-config and ${CHOST}-xml2-config are installed for
each supported build targets.

Fixes: https://bugzilla.gnome.org/show_bug.cgi?id=725635
Fixes: https://bugs.gentoo.org/show_bug.cgi?id=518728