Add long-awaited (:) support for sophisticated dependency checking. We now

Add long-awaited (:) support for sophisticated dependency checking.  We now
have three variables:

EXEC_DEPENDS	- A list of "prog:dir" pairs of other ports this
		  package depends on.  "prog" is the name of an
		  executable.  make will search your $PATH for it and go
		  into "dir" to do a "make all install" if it's not found.
LIB_DEPENDS	- A list of "lib:dir" pairs of other ports this package
		  depends on.  "lib" is the name of a shared library.
		  make will use "ldconfig -r" to search for the
		  library.  Note that lib can be any regular expression,
		  and you need two backslashes in front of dots (.) to
		  supress its special meaning (e.g., use
		  "foo\\.2\\.:${PORTSDIR}/utils/foo" to match "libfoo.2.*").
DEPENDS		- A list of other ports this package depends on being
		  made first.  Use this for things that don't fall into
		  the above two categories.

DEPENDS behaves exactly like before, so old Makefiles will still work
the same.  The two variables are lists of pairs as described above.

For instance, if your program depends on unzip and libjpeg.5.*, use
the following definitions:

EXEC_DEPENDS=	unzip:${PORTSDIR}/archivers/unzip
LIB_DEPENDS=	jpeg\\.5\\.:${PORTSDIR}/graphics/jpeg

gmake:${PORTSDIR}/utils/gmake is automatically added to EXEC_DEPENDS
if USE_GMAKE is defined.

If NO_DEPENDS is defined, the list will just be printed out one by one.