configure: add --disable-rt option

This option disables any attempts in configure to create dependency on
stuff requiring linking to librt.so and libpthread.so, in this case this
means clock_gettime(CLOCK_MONOTONIC, &mt).

We were in need to build curl which doesn't link libpthread.so to avoid
the following bug:
https://sourceware.org/bugzilla/show_bug.cgi?id=16628.