* When using the non-legacy compiler, use the c++ and gcc headers

provided in the gcc_syslibs_devel build feature for building Haiku.
* Simplify declaration of c++ and gcc headers for the legacy compiler -
  in the end we always use the ones living by our source tree anyway.
* Fix a couple of missing local declarations for jam variables, which
  were necessary to avoid a build problem with strace. There are
  probably more bugs like these hiding in our build system files, but
  I'm saving the fix for those to the next commit.
* Add new gcc packages to the HaikuPorts (x86*-)repositories.

build/jam/BuildSetup

@@ -866,6 +866,9 @@ SUPPORTED_PLATFORMS = haiku ;
 # initializations that are required before anything is built for a specific
 # platform can be forced by depending the pseudo target for the platform on the
 # initializations.
+# This is currently used to unpack the external headers from the
+# gcc_syslibs_devel build feature before anything is built for the target
+# platform.
 NotFile $(TARGET_PLATFORM) ;
 NotFile host ;