tcl: bump to version 8.6.1

Turn off building compatibility layers for old/broken versions of
standard functions (strstr, strtoul, strtod) with the assumption that
anything buildroot is using as a standard C library will be good enough
to not have broken behavior.

Signed-off-by: Andrew Ruder <andrew.ruder@elecsyscorp.com>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>