common: compile libcompat as C++

common: compile libcompat as C++

I got errors like these for programs that use libcompat (usually through
libcommon), but are still linked with gcc, rather than g++:

	  CCLD     filter-grammar-test
	/usr/bin/ld: ./.libs/libcommon.a(directory-handle.o):(.data.rel.local.DW.ref.__gxx_personality_v0[DW.ref.__gxx_personality_v0]+0x0): undefined reference to `_
	_gxx_personality_v0'

Automake still links them with gcc, because they don't contain any C++
source directly. Fix that by changing them to be C++ source.

Change-Id: I3eeca3d9af8940795b69f48d306f282ae0b08589
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>