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 5cae5a1def

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

configure

@@ -5496,12 +5496,10 @@ EOF
             x86asmexe=$x86asmexe_probe
             x86asm_type=nasm
             x86asm_debug="-g -F dwarf"
-            X86ASM_DEPFLAGS='-MD $(@:.o=.d)'
         elif check_cmd $x86asmexe_probe --version; then
             x86asmexe=$x86asmexe_probe
             x86asm_type=yasm
             x86asm_debug="-g dwarf2"
-            X86ASMDEP='$(DEPX86ASM) $(X86ASMFLAGS) -M $(X86ASM_O) $< > $(@:.o=.d)'
         fi
         check_x86asm "movbe ecx, [5]" && enable x86asm
     }
@@ -5512,6 +5510,8 @@ EOF
             probe_x86asm $program && break
         done
         disabled x86asm && die "nasm/yasm not found or too old. Use --disable-x86asm for a crippled build."
+        test $x86asm_type = 'nasm' && X86ASM_DEPFLAGS='-MD $(@:.o=.d)'
+        test $x86asm_type = 'yasm' && X86ASMDEP='$(DEPX86ASM) $(X86ASMFLAGS) -M $(X86ASM_O) $< > $(@:.o=.d)'
         X86ASMFLAGS="-f $objformat"
         enabled pic               && append X86ASMFLAGS "-DPIC"
         test -n "$extern_prefix"  && append X86ASMFLAGS "-DPREFIX"