cmake: set CMAKE_POSITION_INDEPENDENT_CODE to ON on Unix

This is needed because we're using the static libraries in a dynamic
library (the Unreal Plugin).

Fixes the error:
relocation R_X86_64_32 against `.bss' can not be used when making a shared object; recompile with -fPIC

cmake/cmake-modules/CommonSetup.cmake

@@ -21,6 +21,7 @@ macro(CommonSetup)
             set(CMAKE_CXX_FLAGS "-std=c++14 -ggdb -Wall -Wextra  -Wstrict-aliasing -Werror -fmax-errors=2 -Wunreachable-code -Wcast-qual -Wctor-dtor-privacy -Wdisabled-optimization -Wformat=2 -Winit-self -Wmissing-include-dirs -Wnoexcept -Wold-style-cast -Woverloaded-virtual -Wredundant-decls -Wshadow -Wstrict-null-sentinel -Wstrict-overflow=5 -Wswitch-default -Wundef -Wno-unused -Wno-variadic-macros -Wno-parentheses -fdiagnostics-show-option ${MAVLINK_OVERRIDES} ${BOOST_OVERRIDES} ${RPC_LIB_DEFINES} -Wl,--no-as-needed -ldl -ldl ${CMAKE_CXX_FLAGS}")
         endif ()
         set(BUILD_PLATFORM "x64")
+        set(CMAKE_POSITION_INDEPENDENT_CODE ON)
 
     ELSE()
         set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -D_WIN32_WINNT=0x0600 /GS /W4 /wd4100 /wd4505 /wd4820 /wd4464 /wd4514 /wd4710 /wd4571 /Zc:wchar_t /ZI /Zc:inline /fp:precise /D_SCL_SECURE_NO_WARNINGS /D_CRT_SECURE_NO_WARNINGS /D_UNICODE /DUNICODE /WX- /Zc:forScope /Gd /EHsc ")