Adjust roken.m4 to allow separate lib and include

In some installations (e.g., Debian), the roken libraries and headers
will not be installed in a common root directory to which /lib and
/include may be appended to find the appropriate library and header
directories, respectively.

Take inspiration from rra-c-util's GSSAPI macros and allow the
specification of separate include and lib directories.  Since there
are now three values to pass to the guts of the checking routine,
pass them in global variables instead of as parameters.

The expected usage would be to set either --with-roken, or both of
--with-roken-libdir and --with-roken-includedir, as in
configure --with-roken-include=/usr/include/heimdal
--with-roken-lib=/usr/lib/x86_64-linux-gnu/heimdal

Change-Id: I6f651ef3f3abf37c92ea81ea1801294ca3dc00b2