Check libraries (Fixes #74)

Adds a new option "--check-libs" to ./update, that checks all libraries in the
current profile.
png: fix rpath to use our zlib library
freetype: fix rpath for zlib
Add bzip2 package
freetype: use our own bzip2
python: use our own zlib
python: use our own bzip2
Lapack: fix rpath issue with blas
libxml2: use our own zlib
hdf5: use our own libz
libxml2: put "make" at the right place

This mistake slipped in while rebasing in one of the previous commits.
Remove hardwired path to "opt"

Now we simply lookup the right path using hashdist.
check-libs: follow symbolic links
Rearrange the systemwide libs
Merge pull request #75 from hashdist/check_libs

Check libraries (Fixes #74)