Previously, this failed if one dependency was a static library compiled
without -fPIC. This is is the case for library libsuitesparseconfig on
Debian which has no dynamic alternative. The error messages read e.g.
```
/usr/bin/ld: /usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/libsuitesparseconfig.a(SuiteSparse_config.o): relocation R_X86_64_PC32 against undefined symbol `malloc@@GLIBC_2.2.5' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: final link failed: Bad value
```
With this patch we change the target_link_libraries call when using
BUILD_SHARED_LIBS=ON. We only pass the dynamic libraries as PUBLIC such
that they will get linked to the library. The static ones are passed as
INTERFACE libraries. This means that they will be automatically linked
to executables which link to the main library. Currently this transitive
linking will only work in the same module as we do not correctly export
libraries. But our build system explicitly links the others ones anyway.
Atgeirr Flø Rasmussen
Arne Morten Kvarving
Markus Blatt
Joakim Hove