doc: clarify that the --with-gmp-* configure options take a path argument
doc: clarify that the --with-gmp-* configure options take a path argument

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl