Without with-boost-libdir, it checks lib64 libx32 lib lib64 in
sysroot/usr/ and set it as boost lib path if it exist, this is
unreliable when build with multilib.
Fixed when switch baselib from lib64 to lib, for example, when we build
qemux86-64 + multilib setting, the baselib is lib64, and when disable
multilib, it would be lib, then we may see the error when
do_configure since lib64 may exist during build source-highlight:
configure: error: Could not find a version of the Boost::Regex library!
