"$llvmlibsdir" in test cases


we are developing a
plugin(http://wiki.llvm.org/Polyhedral_optimization_framework) for
LLVM, and need to find the LLVM libraries directory so we can locate
the loadable module. The testing guide said that we can locate the
libraries directory by the variable "$llvmlibsdir". So i put this
option after opt in the RUN line of test case:
-load=$llvmlibsdir/LLVMPolly.so. but i got:
Error opening '/LLVMPolly.so': /LLVMPolly.so: cannot open shared
object file: No such file or directory
it seems that the variable $llvmlibsdir is empty.

as we also have a lit.site.cfg file:
config.llvm_libs_dir = "/home/ether/work/build/lib"

so i try "-load=$llvm_libs_dir/LLVMPolly.so" to load the module, but
it do not work, too.

any one know how to solve this or some relevant document?

lots of thanks