ruby: don't try to use gmp on the host

When building host-ruby, if gmp seems to be available (for example
when host-gmp is built as a dependency of building the toolchain),
Ruby tries to use it. Unfortunately, it fails to use it because there
is no static version of host-gmp that is built and installed.

Since we anyway don't care much about gmp support in host-ruby, simply
disable it.

This issue was introduced since the bump to Ruby 2.1.0, and this
commit fixes:

  http://autobuild.buildroot.org/results/e19/e197f20bd5ec4aae6d8692d67df4f539a9df33e2/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>