Do not use some compilation flag invalid for clang

-Wstrict-null-sentinel and -rdynamic are invalid flags
for clang compiler.

Signed-off-by: Christophe Courtaut <christophe.courtaut@gmail.com>
Merge pull request #514 from kri5/wip-clang-compilation

Do not use some compilation flag invalid for clang

Reviewed-by: Loic Dachary <loic@dachary.org>
Reviewed-by: Sage Weil <sage@inktank.com>