gcc-source: add comment explaining why a function is Python

(From OE-Core rev: ead8a8e9695ea47ecf8c8eba9cd06cc1a12cc289)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-devtools/gcc/gcc-source.inc

@@ -18,6 +18,8 @@ INHIBIT_DEFAULT_DEPS = "1"
 DEPENDS = ""
 PACKAGES = ""
 
+
+# This needs to be Python to avoid lots of shell variables becoming dependencies.
 python do_preconfigure () {
     import subprocess
     cmd = d.expand('cd ${S} && PATH=${PATH} gnu-configize')