phpExtensions.bcmath: fix Darwin build by setting `-std=gnu17` (#511497)

          #

          # These will be passed as arguments to mkExtension above.

          extensionData = [

            { name = "bcmath"; }

            {

              name = "bcmath";

              env.NIX_CFLAGS_COMPILE = "-std=gnu17";

            }

            {

              name = "bz2";

              buildInputs = [ bzip2 ];