Added m4 macros to check for gcc compiler attributes

Instead of relying on __GNUC__, we now individually check
whether each attribute is supported by the C compiler and set
preprocessor macros accordingly.

Removed similar attribute definitions from wandio.h, as these are
not used at all by libwandio.
Fixed missing AC_LANG_SOURCE from the new m4 macros.

More recent autoconfs are quite strict about this compared with
whatever is in Debian Squeeze.
Replace INCLUDES with AM_CPPFLAGS in Makefile.am's

Again, newer autotools versions complain about using these
outdated terms.
Add subdir-objects option to automake

Seems to be another thing that autotools likes to complain about.
Merge branch 'moregccattrs' into develop