package/ruby: fix build with autoconf >= 2.70

Fix the following build failure raised since bump of autoconf to version
2.71 in commit ecd54b65:

checking whether the C++ compiler works... no
configure: error: in `/home/buildroot/autobuild/instance-3/output-1/build/ruby-3.0.2':
configure: error: C++ compiler cannot create executables

Fixes:
 - http://autobuild.buildroot.org/results/63f32033aff5c800a933d8b1c805be0d9d6abea6



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>