devel/gcc: Check and build GNU sed if not available on build host

Closes #2300

source-builder/config/gcc-common-1.cfg

@@ -165,6 +165,10 @@ URL: 	   http://gcc.gnu.org/
 %build
   build_top=$(pwd)
 
+  %if %{defined _internal_gsed_path}
+    export PATH=%{_internal_gsed_path}/bin:$PATH
+  %endif
+
   %{build_directory}
 
   mkdir -p ${build_dir}