Leaking variables in cmake iox_add_executable and iox_add_library macros

[elBoberido]

Required information

With the introduction of the iox_add_* macros, variables like IOX_STATIC are leaked to the to the environment. This is problematic when more than one library is defined in a CMakeLists.txt or in a nested CMakeLists.txt if e.g. the first library has has set this flag, all other libraries will also use this variable.

It must be checked if the macros or at least some parts of the macros can be replaced with cmake functions. Alternatively it must be checked if these variables can be made unique for each call or if they can be unset at the end of the macro.