configure: OSX/macos: autodetect new_macho value

Use old (=no) if autodetect succeeded and osx ver <= 10, else new.
Override is possible, and now also documented at ./configure --help.

configure

@@ -71,6 +71,12 @@ case $targetos in
     cc=`readlink $cc || echo clang`
     tcc_usrinclude="`xcrun --show-sdk-path`/usr/include"
     DLLSUF=".dylib"
+    case $* in *--config-new_macho*) ;; *)
+        # if new_macho was not specified and (known) ver <= 10, use old (=no)
+        osxver=$(sw_vers -productVersion 2>/dev/null)  # X.Y.Z
+        osxver=${osxver%%.*}  # major version (or empty on sw_vers error)
+        [ "${osxver:-11}" -ge 11 ] || confvars="$confvars new_macho=no"
+    esac
     ;;
   Windows_NT|MINGW*|MSYS*|CYGWIN*)
     mingw32=yes
@@ -351,7 +357,7 @@ Advanced options (experts only):
   --config-backtrace=no    disable stack backtraces (with -run or -bt)
   --config-bcheck=no       disable bounds checker (-b)
   --config-predefs=no      do not compile tccdefs.h, instead just include
-  --config-new_macho=no    Use apple old macho object format
+  --config-new_macho=no|yes Force apple object format (autodetect osx <= 10)
   --config-codesign        Use codesign on apple to sign executables
   --dwarf=x                Use dwarf debug info instead of stabs (x=2..5)
 EOF