Make shlib_loader OBJECT library to prevent multiple compilations

Including sources in targets makes that file being compiled multiple
times. Instead, define an OBJECT library (i.e., it is only compiled, but
not linked), and link it into each target that uses it.

cmake_targets/at_commands/CMakeLists.txt

@@ -102,8 +102,6 @@ find_package(Threads REQUIRED)
 set (FIRMWARE_VERSION "No svn information")
 add_definitions("-DFIRMWARE_VERSION=\"${FIRMWARE_VERSION}\"")
 
-add_boolean_option(LINUX   True  "????")
-
 ##########################
 # NAS LAYER OPTIONS
 ##########################