p4est: revision for gcc
p4est: revision for gcc

Closes #5551.

Signed-off-by: FX Coudert <c329953660db96eae534be5bbf1a735c2baf69b5@gmail.com>

andrewjpage/homebrew-science,dpo/homebrew-science,pditommaso/homebrew-science,philippbayer/homebrew-science,heuermh/homebrew-science,quantumsteve/homebrew-science