librsvg: Add available.mk and tool.mk mk fragments

Rename LIBRSVG_USE_RUST to LIBRSVG_TYPE.

Isolate the LIBRSVG_TYPE logic in available.mk mk fragments in
order that can be used by other librsvg mk (e.g. tool.mk) or by
other package to check the preferred librsvg implementation to be
used. (e.g. devel/pango, fonts/harfbuzz and misc/libreoffice).

Add a tool.mk mk fragment in order to be able to specify librsvg
as a tool dependency honoring user's LIBRSVG_TYPE.
Thanks to <wiz> for suggesting that!

Index: mk/defaults/mk.conf
===================================================================
RCS file: /cvsroot/pkgsrc/mk/defaults/mk.conf,v
retrieving revision 1.302
diff -u -p -r1.302 mk.conf
--- mk/defaults/mk.conf	29 May 2019 12:38:39 -0000	1.302
+++ mk/defaults/mk.conf	5 Jun 2019 15:40:45 -0000
@@ -1293,11 +1293,12 @@ LEAFNODE_GROUP?=	news
 # Default: news
 # Note: default is the same as INN_GROUP
 
-#LIBRSVG_USE_RUST?=	yes
+#LIBRSVG_TYPE?=	rust
 # Used by dependencies of graphics/librsvg to decide if the outdated
 # librsvg-2.40.xx should be used, which is plain C, or if the latest
 # version should be used, which is partially written in rust.
-# Default: yes on operating systems supported by lang/rust
+# Possible: c rust
+# Default: rust on operating systems supported by lang/rust
 
 LINUX_LOCALES?=								\
 	af ar bg br ca cs cy da de el en eo es et eu fi fo fr ga gl	\
Index: graphics/librsvg/available.mk
===================================================================
RCS file: graphics/librsvg/available.mk
diff -N graphics/librsvg/available.mk
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ graphics/librsvg/available.mk	5 Jun 2019 15:40:45 -0000
@@ -0,0 +1,27 @@
+# $NetBSD$
+#
+# This mk fragment define LIBRSVG_TYPE variable used in other librsvg mk
+# fragments.
+#
+# User-settable variables:
+#
+# LIBRSVG_TYPE
+#	Type of used librsvg implementation.
+#
+#	Possible values: c rust
+#	Default: rust on platforms were Rust is supported
+#
+
+.if !defined(LIBRSVG_AVAILABLE_MK)
+LIBRSVG_AVAILABLE_MK=	defined
+
+.include "../../mk/bsd.prefs.mk"
+
+.include "../../lang/rust/platform.mk"
+.if !empty(PLATFORM_SUPPORTS_RUST:M[Yy][Ee][Ss])
+LIBRSVG_TYPE?=	rust
+.else
+LIBRSVG_TYPE?=	c
+.endif
+
+.endif # LIBRSVG_AVAILABLE_MK
Index: graphics/librsvg/buildlink3.mk
===================================================================
RCS file: /cvsroot/pkgsrc/graphics/librsvg/buildlink3.mk,v
retrieving revision 1.48
diff -u -p -r1.48 buildlink3.mk
--- graphics/librsvg/buildlink3.mk	1 Jun 2019 13:55:31 -0000	1.48
+++ graphics/librsvg/buildlink3.mk	5 Jun 2019 15:40:45 -0000
@@ -10,16 +10,16 @@ BUILDLINK_ABI_DEPENDS.librsvg+=		librsvg
 
 .include "../../mk/bsd.fast.prefs.mk"
 
-# default to rust version on platforms where pkgsrc supports lang/rust
-.include "../../lang/rust/platform.mk"
-LIBRSVG_USE_RUST?=	${PLATFORM_SUPPORTS_RUST}
+.include "../../graphics/librsvg/available.mk"
 
-.if ${LIBRSVG_USE_RUST} == "yes"
+.if ${LIBRSVG_TYPE} == "rust"
 BUILDLINK_PKGSRCDIR.librsvg?=		../../graphics/librsvg
 BUILDLINK_API_DEPENDS.librsvg+=		librsvg>=2.41
-.else
+.elif ${LIBRSVG_TYPE} == "c"
 BUILDLINK_PKGSRCDIR.librsvg?=		../../graphics/librsvg-c
 BUILDLINK_API_DEPENDS.librsvg+=		librsvg<2.41
+.else
+PKG_FAIL_REASON+=       "[graphics/librsvg/buildlink3.mk] Invalid value ${LIBRSVG_TYPE} for LIBRSVG_TYPE."
 .endif
 
 .include "../../devel/pango/buildlink3.mk"
Index: graphics/librsvg/tool.mk
===================================================================
RCS file: graphics/librsvg/tool.mk
diff -N graphics/librsvg/tool.mk
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ graphics/librsvg/tool.mk	5 Jun 2019 15:40:45 -0000
@@ -0,0 +1,18 @@
+# $NetBSD$
+#
+# This mk fragment can be included by all packages that need librsvg as a tool
+# dependency (e.g. for rsvg-convert).
+#
+
+.if !defined(LIBRSVG_TOOL_MK)
+LIBRSVG_TOOL_MK=	defined
+
+.include "../../graphics/librsvg/available.mk"
+
+.if ${LIBRSVG_TYPE} == "rust"
+TOOL_DEPENDS+=	librsvg-[0-9]*:../../graphics/librsvg
+.else
+TOOL_DEPENDS+=	librsvg-[0-9]*:../../graphics/librsvg-c
+.endif
+
+.endif # LIBRSVG_TOOL_MK