SConstruct: additional message for the error checking of the python 2.7 headers

SConstruct: additional message for the error checking of the python 2.7 headers

One of the most common errors during the installation of gem5 is the one
related to the failure of the check of the Python2.7 headers in the
SConstruct file. In some cases the headers are correctly installed, but a
wrong selection of the C compiler can make this check fail. This commit
wants to add some useful information this error message.

Change-Id: I0d087ad01c6e4cca3559f23070c37b5c13600962
Reviewed-on: https://gem5-review.googlesource.com/11389
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>