THRIFT-850. build: Perl runtime requires Bit::Vector which may not be installed by...

THRIFT-850. build: Perl runtime requires Bit::Vector which may not be installed by default, but configure does not fail

Attached patch should do the following:

    * check for the existence of the Perl Bit::Vector module
    * warn the user that the Perl library won't be built if Bit::Vector doesn't exist
    * have the WITH_PERL conditional also require Bit::Vector (e.g. don't built the perl libs if the module isn't there)

Patch: Michael Lum

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@987020 13f79535-47bb-0310-9956-ffa450edef68