macros: check for new enough GCC for some macros.
macros: check for new enough GCC for some macros.

mongodbinc-interns/libbson,remicollet/libbson,paulmelnikow/libbson,Machyne/libbson,derickr/libbson,hanumantmk/libbson,ksuarz/libbson,remicollet/libbson,remicollet/libbson,rcsanchez97/libbson,hanumantmk/libbson,ajdavis/libbson,Convey-Compliance/libbson,rcsanchez97/libbson,mongodbinc-interns/libbson,4Second2None/libbson_tmp,bjori/libbson,mapr/libbson,mapr/libbson,alexeyvo/libbson,jqk6/libbson,mapr/libbson,ksuarz/libbson,jqk6/libbson,rcsanchez97/libbson,remicollet/libbson,Convey-Compliance/libbson,derickr/libbson,Convey-Compliance/libbson,4Second2None/libbson_tmp,paulmelnikow/libbson,ksuarz/libbson,jqk6/libbson,derickr/libbson,derickr/libbson,mongodbinc-interns/libbson,paulmelnikow/libbson,rayyang2000/libbson,jqk6/libbson,paulmelnikow/libbson,alexeyvo/libbson,mongodbinc-interns/libbson,hanumantmk/libbson,Machyne/libbson,rayyang2000/libbson,bjori/libbson,paulmelnikow/libbson,ajdavis/libbson,rayyang2000/libbson,4Second2None/libbson_tmp,hanumantmk/libbson,mongodb/libbson,bjori/libbson,mongodb/libbson,Machyne/libbson,alexeyvo/libbson,Machyne/libbson,mongodb/libbson,rayyang2000/libbson,4Second2None/libbson_tmp,ajdavis/libbson