* Flag ${LLVM_INCLUDE_DIRS} as a system include directory

g++ supports a -isystem switch, that can be used to mark a given
directory as a system include directory. Warnings generated by system
include directories are ignored by default.

This commit hides a long list of warnings, like the following one,
generated by llvm header files included from ${LLVM_INCLUDE_DIRS}:

 /usr/lib/llvm-3.7/include/clang/AST/APValue.h:373:44: warning:
   dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
Signed-off-by: Marco Leogrande <marcol@plumgrid.com>

* Fix 'defined but not used' warning

Remove unused function from the USDT probes test.

The warning was:

 tests/cc/test_usdt_probes.cc:59:15: warning:
   ‘size_t countsubs(const string&, const string&)’ defined but not used [-Wunused-function]
Signed-off-by: Marco Leogrande <marcol@plumgrid.com>