libtool is also needed for linking, otherwise downstream
dependencies such as clfft will fail linking against pocl's
libOpenCL.so with errors such as:
```
>> 182 /usr/bin/ld: warning: libltdl.so.7, needed by /usr/local/opt/spack/linux-ubuntu16.04-x86_64/gcc-5.4.0/pocl-0.14-rwo73qzkcxfupndxoz3pcl2s34o5pt6i/lib/libOpenCL.so, not found (try using -rpath or -rpath-link)
>> 183 /usr/local/opt/spack/linux-ubuntu16.04-x86_64/gcc-5.4.0/pocl-0.14-rwo73qzkcxfupndxoz3pcl2s34o5pt6i/lib/libOpenCL.so: undefined reference to `lt_dlinit'
>> 184 /usr/local/opt/spack/linux-ubuntu16.04-x86_64/gcc-5.4.0/pocl-0.14-rwo73qzkcxfupndxoz3pcl2s34o5pt6i/lib/libOpenCL.so: undefined reference to `lt_dlerror'
>> 185 /usr/local/opt/spack/linux-ubuntu16.04-x86_64/gcc-5.4.0/pocl-0.14-rwo73qzkcxfupndxoz3pcl2s34o5pt6i/lib/libOpenCL.so: undefined reference to `lt_dlopen'
>> 186 /usr/local/opt/spack/linux-ubuntu16.04-x86_64/gcc-5.4.0/pocl-0.14-rwo73qzkcxfupndxoz3pcl2s34o5pt6i/lib/libOpenCL.so: undefined reference to `lt_dlsym'
>> 187 collect2: error: ld returned 1 exit status
```
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