build: extract version macros from "nm-version.h" to new header file "nm-version-macros.h"

For libnm library, "nm-dbus-interface.h" contains defines like the D-Bus
paths of NetworkManager. It is desirable to have this header usable without
having a dependency on "glib.h", for example for a QT application. For that,
commit c0852964a8 removed that dependancy.

For libnm-glib library, the analog to "nm-dbus-interface.h" is
"NetworkManager.h", and the same applies there. Commit
159e827a72 removed that include.
However, that broke build on PackageKit [1] which expected to get the
version macros by including "NetworkManager.h". So at least for libnm-glib,
we need to preserve old behavior so that a user including
"NetworkManager.h" gets the version macros, but not "glib.h".

Extract the version macros to a new header file "nm-version-macros.h".
This header doesn't include "glib.h" and can be included from
"NetworkManager.h". This gives as previous behavior and a glib-free
include.

For libnm we still don't include "nm-version-macros.h" to "nm-dbus-interface.h".
Very few users will actually need the version macros, but not using
libnm.
Users that use libnm, should just include (libnm's) "NetworkManager.h" to
get all headers.
As a special case, a user who doesn't want to use glib/libnm, but still
needs both "nm-dbus-interface.h" and "nm-version-macros.h", can include
them both separately.

[1] https://github.com/hughsie/PackageKit/issues/85

Fixes: 4545a7fe96

src/dhcp-manager/Makefile.am

@@ -5,6 +5,7 @@ nm_dhcp_helper_SOURCES = nm-dhcp-helper.c
 nm_dhcp_helper_CPPFLAGS = \
 	$(GLIB_CFLAGS) \
 	-I$(top_srcdir)/include \
+	-I${top_builddir}/include \
 	-I$(top_srcdir)/libnm-core \
 	-I$(top_builddir)/libnm-core \
 	-DG_LOG_DOMAIN=\""nm-dhcp-helper"\" \

src/dhcp-manager/tests/Makefile.am

@@ -1,5 +1,6 @@
 AM_CPPFLAGS = \
 	-I$(top_srcdir)/include \
+	-I${top_builddir}/include \
 	-I${top_srcdir}/libnm-core \
 	-I${top_builddir}/libnm-core \
 	-I$(top_srcdir)/src/dhcp-manager \