build: fix dtrace-related warnings

Fix the warnings when compiling and linking the probes files by
calling dtrace with all the -W flags removed from CFLAGS (since dtrace
generates bad C code), and with CC set to "libtool --mode=compile ..."
(so that it will output a proper .lo file and libtool won't warn when
linking it into the .la).

https://bugzilla.gnome.org/show_bug.cgi?id=693335
update .gitignores