install library and header

[christianrauch]

This extends the CMake instructions to install the library and header files for Fusion. It also sets enables PIC..

The definition of the BUILD_INTERFACE and INSTALL_INTERFACE in target_include_directories fixes issues such as:

CMake Error in [...]/_deps/madgwick_fusion-src/Fusion/CMakeLists.txt:
  Target "Fusion" INTERFACE_INCLUDE_DIRECTORIES property contains path:

    "[...]/_deps/madgwick_fusion-src/Fusion/."

  which is prefixed in the build directory.

when the repo is used via CMake's FetchContent.

Enabling PIC fixes issues such as:

/usr/bin/ld: ../_deps/madgwick_fusion-build/Fusion/libFusion.a(FusionAhrs.c.o): warning: relocation against `fusionAhrsDefaultSettings' in read-only section `.text'
/usr/bin/ld: ../_deps/madgwick_fusion-build/Fusion/libFusion.a(FusionAhrs.c.o): relocation R_X86_64_PC32 against symbol `fusionAhrsDefaultSettings' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: final link failed: bad value