configure: fix assignment of assembler specific flags

If the first assembler to be probed is an old nasm build, X86ASM_DEPFLAGS
will be set and remain so after yasm is ultimately used as fallback.
This results in yasm being called with said nasm specific flags and failing
during actual object assembly but not with configure sanity checks.

Regression since 5cae5a1d

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: James Almer <jamrial@gmail.com>