RFR(S): 8146639: Fix detection of Cups headers during configuration

[Volker Simonis]

Hi,

can somebody please review and sponsor (needs regeneration of
generated-configure.sh because of closed sources) the following small
fix:

http://cr.openjdk.java.net/~simonis/webrevs/2016/8146639/
https://bugs.openjdk.java.net/browse/JDK-8146639

Currently, if the location of the Cups headers is passed to configure
via the --with-cups or --with-cups-inlcude option, the locations are
not checked to really contain the required Cups header files. This
will lead to a compilation error, if there are no Cups header files in
the default include path.

Even worse, if the user wants to specify special versions of the Cups
header files via the --with-cups or --with-cups-inlcude options but
mis-types the path while there exist Cups header files in the standard
include path, the standard headers will be used silently (because the
location given by --with-cups or --with-cups-inlcude isn't checked and
rejected.)

Thank you and best regards,
Volker