Hack to get posix build partially working

Created a dummy build_git_version.h so I did not have to figure out
how to generate it.

Commented out topic_listener.cpp from src/systemcmds/CMakeLists.txt
because it depends on a generated cpp file.

Link doesn't work mut many modules build

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

CMakeLists.txt

@@ -110,6 +110,7 @@ set(package-contact "px4users@googlegroups.com")
 #
 
 # set module path
+message("OS = ${OS}")
 list(APPEND CMAKE_MODULE_PATH ${CMAKE_SOURCE_DIR}/cmake)
 list(APPEND CMAKE_MODULE_PATH ${CMAKE_SOURCE_DIR}/cmake/${OS})