configure: remove -Wdeclaration-after-statement from the parameter for GCC

Current sheepdog employes gnu99 style. But
-Wdeclaration-after-statement is still passed to GCC so interleaving
declarations and statements is not allowed. This patch remove it.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>