tinyhttpd: do not link against pthread

Original patch for tinyhttpd disables pthread support from the code
but does not disable it from makefile.
This patch disables pthread support altogether

Fixes:
http://autobuild.buildroot.net/results/2e61a80fcbddd62c76e0406c34ab44d14a4b5c79



Signed-off-by: Gilles Talis <gilles.talis@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>