Fix dependancy with llvm version

Compiling ldc2-9999 against llvm-3.1 was failing with the following error:

/usr/bin/x86_64-pc-linux-gnu-g++   -O2 -ftree-vectorize -march=native -mtune=native -pipe     -Wl,-O1 -Wl,--as-needed CMakeFiles/ldc2.dir/driver/cl_options.cpp.o CMakeFiles/ldc2.dir/driver/configfile.cpp.o CMakeFiles/ldc2.dir/driver/toobj.cpp.o CMakeFiles/ldc2.dir/driver/linker.cpp.o CMakeFiles/ldc2.dir/driver/main.cpp.o  -o bin/ldc2 -rdynamic lib/libldc.a -lconfig++ -L/usr/lib64/llvm  -lpthread -lffi -ldl -lm /usr/lib64/llvm/libLLVMAsmParser.a /usr/lib64/llvm/libLLVMTableGen.a /usr/lib64/llvm/libLLVMInstrumentation.a /usr/lib64/llvm/libLLVMipo.a /usr/lib64/llvm/libLLVMVectorize.a /usr/lib64/llvm/libLLVMLinker.a /usr/lib64/llvm/libLLVMArchive.a /usr/lib64/llvm/libLLVMBitReader.a /usr/lib64/llvm/libLLVMBitWriter.a /usr/lib64/llvm/libLLVMX86AsmParser.a /usr/lib64/llvm/libLLVMX86CodeGen.a /usr/lib64/llvm/libLLVMSelectionDAG.a /usr/lib64/llvm/libLLVMAsmPrinter.a /usr/lib64/llvm/libLLVMMCParser.a /usr/lib64/llvm/libLLVMCodeGen.a /usr/lib64/llvm/libLLVMScalarOpts.a /usr/lib64/llvm/libLLVMInstCombine.a /usr/lib64/llvm/libLLVMTransformUtils.a /usr/lib64/llvm/libLLVMipa.a /usr/lib64/llvm/libLLVMAnalysis.a /usr/lib64/llvm/libLLVMX86Disassembler.a /usr/lib64/llvm/libLLVMX86Desc.a /usr/lib64/llvm/libLLVMX86Info.a /usr/lib64/llvm/libLLVMX86AsmPrinter.a /usr/lib64/llvm/libLLVMX86Utils.a /usr/lib64/llvm/libLLVMCppBackendCodeGen.a /usr/lib64/llvm/libLLVMCppBackendInfo.a /usr/lib64/llvm/libLLVMTarget.a /usr/lib64/llvm/libLLVMCore.a /usr/lib64/llvm/libLLVMMC.a /usr/lib64/llvm/libLLVMObject.a /usr/lib64/llvm/libLLVMSupport.a -ldl 
`.text._ZN4llvm13RGPassManagerD2Ev' referenced in section `.text._ZN4llvm13RGPassManagerD1Ev[_ZN4llvm13RGPassManagerD1Ev]' of /usr/lib64/llvm/libLLVMAnalysis.a(RegionPass.o): defined in discarded section `.text._ZN4llvm13RGPassManagerD2Ev[_ZN4llvm13RGPassManagerD5Ev]' of /usr/lib64/llvm/libLLVMAnalysis.a(RegionPass.o)
collect2: error: ld returned 1 exit status
make[2]: *** [bin/ldc2] Error 1