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

This also fixes a typo that prevented --with-roken=internal from
functioning as intended.

Change-Id: I6f651ef3f3abf37c92ea81ea1801294ca3dc00b2
Reviewed-on: http://gerrit.openafs.org/11473
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: D Brashear <shadow@your-file-system.com>
Tested-by: D Brashear <shadow@your-file-system.com>
Allow building with MIT krb5 and external roken

That is, an external roken which is part of a heimdal distribution,
with full headers and libraries, most notably krb5.h and libkrb5.

This adjusts the ordering of file- and module-specific compiler and
linker arguments so that the more specific arguments are able to
take precedence.  For general flags arguments, such as enabling
or disabling warnings or features, the more-specific settings should
come last, so as to override the flags set by default.  However,
for arguments that affect a global search list (e.g., for headers
or libraries),  the more-specific arguments must come first, so
as to be at the beginning of the search list.

We presently use per-file CFLAGS for both warning-type flags and
preprocessor (i.e., include path) type flags, so add an additional
file-specific setting for CPPFLAGS, which comes at the beginning of
the compiler invocation.

At present, MODULE_CFLAGS are essentially only used for preprocessor
functionality, so treat them as CPPFLAGS and put them right after
the per-file CPPFLAGS.  (It might be cleaner to rename them to
MODULE_CPPFLAGS, but that would be more churn than is needed.  If
such a distinction turns out to be necessary, it can be done at a
later date.)  Likewise the MODULE_LDFLAGS are generally being used
to affect the library search path, so put them early as well.

Make the necessary Makefile changes to use these new features to
allow building with MIT krb5 and external roken: put KRB5_CPPFLAGS
in per-file CPPFLAGS, and put LDFLAGS_KRB5 in MODULE_LDFLAGS for
aklog.

Change-Id: I1091223b3b75c782b39b9e189bdd47e52ebefae2
Reviewed-on: http://gerrit.openafs.org/11474
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: D Brashear <shadow@your-file-system.com>
Tested-by: D Brashear <shadow@your-file-system.com>