Add versioned NM_DEPRECATED_IN_* and NM_AVAILABLE_IN_* macros, and tag
new/deprecated functions accordingly. (All currently-deprecated
functions are assumed to have been deprecated in 0.9.10.)

Add NM_VERSION_MIN_REQUIRED and NM_VERSION_MAX_ALLOWED macros which
can be set to determine which versions will cause warnings.

With the current settings, external consumers of the
libnm-util/libnm-glib APIs will have MIN_REQUIRED and MAX_ALLOWED both
set to NM_VERSION_0_9_8 by default, meaning they will get warnings
about functions added in 0.9.10. NM internally sets
NM_VERSION_MAX_ALLOWED to NM_VERSION_NEXT_STABLE to ensure that it is
always allowed to use all APIs.

Unfortunately, $(AM_CPPFLAGS) gets overridden by per-target _CPPFLAGS
variables, which $(INCLUDES) did not, so this requires some additional
changes.

In most places, I have just gotten rid of the per-target _CPPFLAGS
variables; in directories with a single target, the per-target
variable is unnecessary, and in directories with multiple targets, the
per-target variable is often undesirable, since it forces some files
to be compiled twice, even though there ends up being no difference
between the two files.

Relative paths in makefiles have different anchor than the current shell
directory.

Also, we can get rid of tools/manpages-docbook.xsl and just specify
the canonical URL directly.

It makes the initializations more explicit.
G_VALUE_INIT is available since glib 2.30, and NM now require glib >= 2.32

This was imported to get a bugfix in glib 2.31.5; since we require
2.32 now, we can use the system glib-mkenums.