g_clear_pointer: work around gcc helpfulness
g_clear_pointer: work around gcc helpfulness

gcc gets upset when we do "((GDestroyNotify) destroy) (_p)" because
it's non-portable. But we don't care; we already know glib wouldn't
work on any platform where different pointer types have different
calling conventions. So tweak the code to avoid the warning.

https://bugzilla.gnome.org/show_bug.cgi?id=674634

endlessm/glib,lukasz-skalski/glib,gale320/glib,johne53/MB3Glib,lukasz-skalski/glib,cosimoc/glib,tamaskenez/glib,johne53/MB3Glib,tchakabam/glib,krichter722/glib,Distrotech/glib,cention-sany/glib,tamaskenez/glib,tchakabam/glib,ieei/glib,krichter722/glib,cosimoc/glib,lukasz-skalski/glib,cosimoc/glib,MathieuDuponchelle/glib,endlessm/glib,endlessm/glib,mzabaluev/glib,ieei/glib,MathieuDuponchelle/glib,ieei/glib,endlessm/glib,johne53/MB3Glib,tchakabam/glib,MathieuDuponchelle/glib,cention-sany/glib,djdeath/glib,mzabaluev/glib,djdeath/glib,tamaskenez/glib,Distrotech/glib,mzabaluev/glib,endlessm/glib,tchakabam/glib,cention-sany/glib,ieei/glib,MathieuDuponchelle/glib,Distrotech/glib,djdeath/glib,johne53/MB3Glib,djdeath/glib,gale320/glib,ieei/glib,djdeath/glib,johne53/MB3Glib,tamaskenez/glib,tamaskenez/glib,Distrotech/glib,cosimoc/glib,krichter722/glib,gale320/glib,krichter722/glib,mzabaluev/glib,cention-sany/glib,gale320/glib,lukasz-skalski/glib,cosimoc/glib,johne53/MB3Glib,tchakabam/glib,krichter722/glib,Distrotech/glib,lukasz-skalski/glib,cention-sany/glib,mzabaluev/glib,MathieuDuponchelle/glib,gale320/glib