Enable -fPIC in gcc/clang

This is to prevent errors like this one, while linking liboctomath.a against another user shared library:

```
/usr/bin/ld: ../../lib/liboctomath.a(Vector3.cpp.o): relocation R_X86_64_32 against `.rodata.str1.1' can not be used when making a shared object; recompile with -fPIC
../../lib/liboctomath.a: error adding symbols: Bad value
```

CMakeLists.txt

@@ -2,6 +2,7 @@ CMAKE_MINIMUM_REQUIRED(VERSION 2.8.8)
 PROJECT( octomap-distribution )
 
 ENABLE_TESTING()  # enable CTest environment of subprojects
+set(CMAKE_POSITION_INDEPENDENT_CODE ON)  # enables -fPIC in applicable compilers (required to avoid link errors in some cases)
 
 option(BUILD_OCTOVIS_SUBPROJECT "Build targets from subproject octovis" ON)
 option(BUILD_DYNAMICETD3D_SUBPROJECT  "Build targets from subproject dynamicEDT3D" ON)