backport world builder simplify cmake -fPIC

[MFraters]

A follow up to #5676 and #5669. @bangerth suggested that the -fPIC flags could also be removed.

The -fPIC flag should be automatically set by cmake, so there is no need to specify it manually.

This is a backport of GeodynamicWorldBuilder/WorldBuilder#736 into aspect.

[tjhei]

/usr/bin/ld: lib/libWorldBuilder.a(unity_0_cxx.cxx.o): relocation R_X86_64_PC32 against symbol `_ZTVN12WorldBuilder17CoordinateSystems9CartesianE' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: final link failed: bad value

[MFraters]

Yes, it seems that -fPIC is actually needed to get it to work with aspect, but also in the world builder repository. Maybe I need to set something for cmake that it adds it when needed.

[bangerth]

@MFraters Would you mind updating this accordingly?

[MFraters]

@MFraters Would you mind updating this accordingly?

Done!