gcc2: Add missing library and header search paths

* For /boot/common[/non-packaged] the standard include paths should be
  develop/headers/ as well instead of include/.
* Build the standard library search paths (the system and common ones)
  into the compiler as well. This makes setting the BELIBRARIES
  environment variable superfluous.
* The obsolete search paths (/boot/common[/non-packaged]/include/ and
  /boot/common/lib/ respectively) are still built in as well. They will
  be removed after a grace period, when all packages have been adjusted
  accordingly.
Bump version to 130425