Fix implicit function declarations, bug 660531

Fix numerous vinagre_utils_* and vinagre_dirs_* implicit function
declarations by generating a header (vinagre/vinagre-vala.h) from the
vala source and including it in half the .c files in the source tree.

Add the vinagre-vala.h to dist_noinst_DATA to make sure it goes in the
release tarballs, and to BUILT_SOURCES to ensure that it is generated
before the C sources that #include it are compiled. Add
-I$(top_srcdir)/vinagre to CPPFLAGS so that vinagre-vala.h is picked up
in out-of-tree builds.

Unfortunately, automake does not support per-target VALAFLAGS. We cannot
simply add a "--header vinagre/vinagre-vala.h" option to AM_VALAFLAGS
or VALAFLAGS because if we ever use a .vala file in another target (such
as one of the vinagre plugins), vinagre-vala.h will get clobbered.

The only way to prevent valac from running multiple times and safely
support parallel make without adding a ticking time bomb that could
clobber vinagre-vala.h is to manually write a stamp ...