configure: probe_cc: use separate variable for linker output flag

Some tools use different command line syntax for specifying output
when compiling and linking.  To accomodate these, separate variables
must be used.  No currently supported compilers/linkers are affected
by the change.
Signed-off-by: Mans Rullgard <mans@mansr.com>

configure

@@ -2129,7 +2129,7 @@ probe_cc(){
     pfx=$1
     _cc=$2
 
-    unset _type _ident _cc_c _cc_e _cc_o _flags _cflags _ldflags
+    unset _type _ident _cc_c _cc_e _cc_o _ld_o _flags _cflags _ldflags
     unset _depflags _DEPCMD _DEPFLAGS
     _flags_filter=echo
 
@@ -2291,7 +2291,7 @@ probe_cc ld "$ld"
 ldflags_filter=$_flags_filter
 add_ldflags $_flags $_ldflags
 test "$cc_type" != "$ld_type" && add_ldflags $cc_ldflags
-LD_O=${_cc_o-$LD_O}
+LD_O=${_ld_o-$LD_O}
 
 if [ -z "$CC_DEPFLAGS" ] && [ "$dep_cc" != "$cc" ]; then
     probe_cc depcc "$dep_cc"