libxl: fix api check Makefile

libxl: fix api check Makefile

Touch the libxl.api-ok stamp file, and unconditionally put in place
the new _libxl.api-for-check.  This avoids needlessly rerunning the
preprocessor on libxl.h each time we call "make".

Ensure that _libxl.api-for-check gets the CFLAGS used for xl, so that
if it is asked for in a standalone make run it can find xentoollog.h.

Remove *.api-ok on clean.

Also fix .gitignore.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>