Manually choose linker for automake

Automake will choose g++ as linker if a c++ source file only appears in
the SOURCES list conditionally. This will prevent lttng-tools compiling
without a c++ compiler. Thus we should manually choose a proper linker.

http://permalink.gmane.org/gmane.comp.sysutils.automake.bugs/4184
Send probe information from enable-event command to sessiond

This version should not break any API/ABI