configure: Consider $ac_tool_prefix for pkg-config

Pacemaker fails to cross build from source, because it uses the build
architecture pkg-config by searching for it with AC_PATH_PROG rather
than AC_PATH_TOOL or even better PKG_PROG_PKG_CONFIG. The latter two
macros do consider $ac_tool_prefix which is required for cross building.

After applying the attached patch, Pacemaker still fails to cross build
due to its use of help2man. There is no obvious solution for that
problem, but I think the attached patch is still useful, as it makes the
help2man problem visible and moves pacemaker's configure.ac to a more
correct one. Please consider applying it.

Patch from https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=865048