Canonicalize $srcdir in configure.ac

For krb5.conf include/includedir we want to reject non-absolute paths,
but then we need to make sure that we use absolute paths in the tests,
otherwise they fail.  Of course ./configure has been defaulting to
relative paths for $srcdir and $objdir.  This commit canonicalizes
$srcdir; eventually, no doubt, we'll have to canonicalize $objdir too.

configure.ac

@@ -7,6 +7,8 @@ AC_CONFIG_SRCDIR([kuser/kinit.c])
 AC_CONFIG_HEADERS(include/config.h)
 AC_CONFIG_MACRO_DIR([cf])
 
+srcdir=`cd "$srcdir" && pwd || echo "$srcdir"`
+
 AM_INIT_AUTOMAKE([foreign 1.11])
 AM_MAINTAINER_MODE