make -lrt linking optional

On Android there is no standalone rt library and relevant symbols are provided
by libc (bionic).

introspection/CMakeLists.txt

@@ -6,8 +6,14 @@ include_directories(${CMAKE_SOURCE_DIR}/src/cc/api)
 include_directories(${CMAKE_SOURCE_DIR}/src/cc/libbpf/include/uapi)
 
 option(INSTALL_INTROSPECTION "Install BPF introspection tools" ON)
+option(BPS_LINK_RT "Pass -lrt to linker when linking bps tool" ON)
+
+set(bps_libs_to_link bpf-static elf z)
+if(BPS_LINK_RT)
+list(APPEND bps_libs_to_link rt)
+endif()
 
 add_executable(bps bps.c)
-target_link_libraries(bps bpf-static elf rt z)
+target_link_libraries(bps ${bps_libs_to_link})
 
 install (TARGETS bps DESTINATION share/bcc/introspection)