webkit: Fix build issue with gcc-4.6 and above

This fixes the "Unknown parameter for tags/attrs" build error.
Backported from commit 88e08c43d0200a4b06a298b7d2541965eebc0afe

[PATCH] 2011-04-17  Thierry Reding
<thierry.reding@avionic-design.de>

  Reviewed by Adam Barth.

  Fix build with GCC 4.6.

  * dom/make_names.pl: Execute preprocessor without the -P option. The
  preprocessor in GCC 4.6 eats empty lines, effectively breaking the
  parsing performed by this script. Dropping the -P option when invoking
  the preprocessor keeps the empty lines but as a side-effect also adds
  additional linemarkers.

  From the cpp manpage:

    -P  Inhibit generation of linemarkers in the output from the
        preprocessor. This might be useful when running the preprocessor
        on something that is not C code, and will be sent to a program
        which might be confused by the linemarkers.

  The linemarkers are not problematic, however, because the script
  properly handles them by ignoring all lines starting with a #.

Signed-off-by: Valentine Barshak <gvaxon@gmail.com>
Tested-by: Samuel Martin <s.martin49@gmail.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>