What is the idiomatic way in CMAKE to add the -fPIC compiler option?

You can set the position independent code property on all targets:

set(CMAKE_POSITION_INDEPENDENT_CODE ON)

or in a specific library:

add_library(lib1 lib1.cpp)
set_property(TARGET lib1 PROPERTY POSITION_INDEPENDENT_CODE ON)

Reference:

• CMAKE_POSITION_INDEPENDENT_CODE
• cmake build system: “Compatible Interface Properties”