[perl #90122] Make h2ph correctly search gcc include directories

[perl #90122] Make h2ph correctly search gcc include directories

System header conversion with "h2ph -a" is currently broken on Ubuntu
Natty and Oneiric (unless the gcc-multilib package is installed for
backward compatibility), resulting in things like

 # perl -e 'require "syscall.ph"'
 Can't locate asm/unistd.ph in @INC [...]

This happens because Ubuntu has switched to a 'multiarch' setup, see
<https://wiki.ubuntu.com/MultiarchSpec> for details.

The asm subdirectory isn't in $Config{usrinc} anymore: /usr/include/asm
is now /usr/include/x86_64-linux-gnu/asm. (The third component of the
new path varies with the actual architecture.)

gcc --print-search-dirs doesn't really tell anything about where gcc
looks for the include directories, it was just used to find the gcc
internal directory prefix.

Parse the output of "gcc -v -E" instead, and append $Config{usrinc}
for safety. Duplicates shouldn't matter.

The h2ph "-a" switch isn't currently tested automatically, and that
seems nontrivial to do portably. Manual testing was done with

 # mkdir ttt
 # ./perl -Ilib ./utils/h2ph -a -d $(pwd)/ttt syscall.h

The gcc invocation has been tested to work with gcc 4.6, 4.1, and 3.3.

http://bugs.debian.org/625808
https://bugs.launchpad.net/bugs/777903