Add cmake option to enable PIC

[mockbutler]

Allows library to be used in the creation of dynamic libraries. Not enabled by default.

[nakarlsson]

ping @hefloryd

[hefloryd]

-fPIC is added automatically by cmake if BUILD_SHARED_LIBS is enabled. Currently of course a static library is always built, so you will also have to remove STATIC from the cmakelist, but I guess you would need to do that anyway in order to build a shared library.

Note that PR #343 modernizes the cmakelist and lets you build a shared library without editing the cmakelists (just by setting BUILD_SHARED_LIBS). Please try it out if you have the time.

[mockbutler]

@hefloryd For my purposes I want to build the static library with -fPIC. This library is then linked into another project which produces a dynamic library. The second project provides wrapper code for a managed language. So linking against an -fPIC static library simplifies the distribution to a single native library, precluding dynamic library version mismatch.

This use case is atypical, but I didn't think unique. So I thought might be useful to support this almost hybrid target. There are other ways to achieve this, but using CMake allows the broad platform support.

[lepalom]

I agree with @mockbutler and I would like to add that it's a typical case to use SOEM and build a shared library to provide a wrapper for a managed language.

Also, the BUILD_SHARED_LIBS is referred to build a shared lib when you have add_library, so it's not clear why to set this parameter to add the -PIC flag.

[hefloryd]

Your use-case seems reasonable but would it not also be sufficient to configure SOEM with -DCMAKE_POSITION_INDEPENDENT_CODE=ON? There are many cmake flags that a user might wish to set and I'm not sure the best option is to wrap them in an SOEM option.