* 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/repositories/HaikuPorts/x86

@@ -44,7 +44,9 @@ RemotePackageRepository HaikuPorts
 	freetype-2.5.2-3
 	freetype_devel-2.5.2-3
 	gawk-4.1.0-2
-	gcc-4.8.3_2014_05_28-2
+	gcc-4.8.3_2014_07_26-1
+	gcc_syslibs-4.8.3_2014_07_26-1
+	gcc_syslibs_devel-4.8.3_2014_07_26-1
 	gettext-0.18.1.1-5
 	gettext_libintl-0.18.1.1-5
 	git-1.8.3.4-2
@@ -177,6 +179,7 @@ RemotePackageRepository HaikuPorts
 	freetype_x86_gcc2-2.5.2-3
 	freetype_x86_gcc2_devel-2.5.2-3
 	gcc_x86_gcc2-2.95.3_2014_07_26-1
+	gcc_x86_gcc2_syslibs_devel-2.95.3_2014_07_26-1
 	gettext_x86_gcc2-0.18.1.1-5
 	gettext_x86_gcc2_libintl-0.18.1.1-5
 	glu_x86_gcc2-9.0.0-2
@@ -244,7 +247,7 @@ RemotePackageRepository HaikuPorts
 	fossil
 	freetype
 	gawk
-#	gcc
+	gcc
 	gcc_x86_gcc2
 	gettext
 	git