configure: fix symbol prefix detection

The symbol prefix check would incorrectly detect a bogus prefix in
circumstances where sanitizers instrument the build, like when
configuring with the clang-asan toolchain where it would detect the
prefix as __odr_asan_gen_, which is obviously wrong.

To fix this, adjust the prefix detection to only detect a
one-character prefix, which is the only case that matters
anywhere right now.

configure

@@ -6137,9 +6137,11 @@ enabled pic && enable_weak_pic
 test_cc <<EOF || die "Symbol mangling check failed."
 int ff_extern;
 EOF
-sym=$($nm $TMPO | awk '/ff_extern/{ print substr($0, match($0, /[^ \t]*ff_extern/)) }')
+sym=$($nm $TMPO | awk '/[ \t]+[^ \t]{0,1}ff_extern$/{ print substr($0, match($0, /[^ \t]{0,1}ff_extern$/)) }')
 extern_prefix=${sym%%ff_extern*}
 
+log "Symbol prefix detected as: '${extern_prefix}'"
+
 ! disabled inline_asm && check_inline_asm inline_asm '"" ::'
 
 check_cc pragma_deprecated "" '_Pragma("GCC diagnostic push") _Pragma("GCC diagnostic ignored \"-Wdeprecated-declarations\"")'