Use cross-build compatible macro for finding pkg-config.

The package uses AC_PATH_PROG to discover the location of pkg-config(1). This
macro fails to select the correct version to support cross-compilation.

This patch changes it to use AC_PATH_TOOL.

Refer to https://bugs.debian.org/884798 for details.

Fixed-Lintian-Tags: autotools-pkg-config-macro-not-cross-compilation-safe
Patch-Name: ac-path-pkgconfig
