Using pkg-config

Like other GNOME libraries, libsoup uses pkg-config to provide compiler options. The package name is "libsoup-2.4". So in your configure script, you might specify something like:

PKG_CHECK_MODULES(LIBSOUP, [libsoup-2.4 >= 2.26])
AC_SUBST(LIBSOUP_CFLAGS)
AC_SUBST(LIBSOUP_LIBS)

The "2.4" in the package name is the "API version" (indicating "the version of the libsoup API that first appeared in version 2.4") and is essentially just part of the package name.

If you are using any of the GNOME-specific features of libsoup (such as automatic proxy configuration), you must require "libsoup-gnome-2.4" instead:

PKG_CHECK_MODULES(LIBSOUP, [libsoup-gnome-2.4 >= 2.26])
AC_SUBST(LIBSOUP_CFLAGS)
AC_SUBST(LIBSOUP_LIBS)

You can also make libsoup-gnome an optional dependency:

PKG_CHECK_MODULES(LIBSOUP_GNOME,
		  [libsoup-gnome-2.4 >= 2.26],
		  [LIBSOUP_CFLAGS="$LIBSOUP_GNOME_CFLAGS"
		   LIBSOUP_LIBS="$LIBSOUP_GNOME_LIBS"
		   AC_DEFINE(HAVE_LIBSOUP_GNOME, 1, [Have libsoup-gnome])],
		  [PKG_CHECK_MODULES(LIBSOUP, [libsoup-2.4 >= 2.26])])
AC_SUBST(LIBSOUP_CFLAGS)
AC_SUBST(LIBSOUP_LIBS)

This will allow the application to be built with either plain libsoup or with libsoup-gnome, and it will define the C preprocessor symbol HAVE_LIBSOUP_GNOME if libsoup-gnome features are available.

Headers

Code using libsoup should do:

#include <libsoup/libsoup.h>

or, for libsoup-gnome:

#include <libsoup/libsoup-gnome.h>

Including individual headers besides the two main header files is not recommended. You may include both libsoup.h and libsoup-gnome.h (though this is not required; the latter automatically includes the former).