Jon Turney authored Mar 06, 2017 and Adam Jackson committed Mar 14, 2017

The linemarkers in the preprocessor output from gcc5 on Cygwin have
canonicalized paths to included files (e.g. xserver/build/../include/misc.h
is canonicalized to xserver/build/include/misc.h). (see gcc svn rev 210264,
which causes the transformation performed by -fcanonical-system-headers to
be applied to all include pathnames)

These canonicalized paths won't match $topdir, so sdksyms doesn't look at
the contents of those headers for sdk exported symbols.

Workaround this by canonicalizing all the paths we consider, using readlink.

v2:
Keep a cache of readlink results so it isn't quite so dreadfully slow.

Reviewed-by: Adam Jackson <ajax@redhat.com>