Configure: allow some file extensions to be overridden by target config.

Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6487)

Configure

@@ -1111,8 +1111,9 @@ $target{exe_extension}=".exe" if ($config{target} eq "DJGPP"
 $target{exe_extension}=".pm"  if ($config{target} =~ /vos/);
 
 ($target{shared_extension_simple}=$target{shared_extension})
-    =~ s|\.\$\(SHLIB_VERSION_NUMBER\)||;
-$target{dso_extension}=$target{shared_extension_simple};
+    =~ s|\.\$\(SHLIB_VERSION_NUMBER\)||
+    unless defined($target{shared_extension_simple});
+$target{dso_extension}//=$target{shared_extension_simple};
 ($target{shared_import_extension}=$target{shared_extension_simple}.".a")
     if ($config{target} =~ /^(?:Cygwin|mingw)/);