guide: Document TOOL_DEPENDS

Document TOOL_DEPENDS and clarify the differencies with BUILD_DEPENDS.
Adjust previous BUILD_DEPENDS examples (all of them were actually
TOOL_DEPENDS).

TODO: (DO NOT COMMIT IT AS-IS!): find a good BUILD_DEPENDS example.

Index: creating.xml
===================================================================
RCS file: /cvsroot/pkgsrc/doc/guide/files/creating.xml,v
retrieving revision 1.27
diff -u -p -r1.27 creating.xml
--- creating.xml	4 Jun 2018 23:55:00 -0000	1.27
+++ creating.xml	4 May 2019 12:54:11 -0000
@@ -43,13 +43,22 @@ created first. The <filename>buildlink3.
 version of the dependency and where it can be found in pkgsrc. This line
 should be placed in the third paragraph. If the dependency is only
 needed for building the package, but not when using it, use
-<varname>BUILD_DEPENDS</varname> instead of <varname>DEPENDS</varname>.
+<varname>TOOL_DEPENDS</varname> or <varname>BUILD_DEPENDS</varname>
+instead of <varname>DEPENDS</varname>.
+The difference between <varname>TOOL_DEPENDS</varname> and
+<varname>BUILD_DEPENDS</varname> occurs when cross-compiling:
+<varname>TOOL_DEPENDS</varname> are <emphasis>native</emphasis>
+packages, i.e. packages for the architecture where the package
+is built;
+<varname>BUILD_DEPENDS</varname> are <emphasis>target</emphasis>
+packages, i.e. packages for the architecture for which the package
+is built.
 Your package may then look like this:</para>
 
 <programlisting>
 [...]
 
-BUILD_DEPENDS+= libxslt-[0-9]*:../../textproc/libxslt
+TOOL_DEPENDS+=  libxslt-[0-9]*:../../textproc/libxslt
 DEPENDS+=       screen-[0-9]*:../../misc/screen
 DEPENDS+=       screen>=4.0:../../misc/screen
 
Index: fixes.xml
===================================================================
RCS file: /cvsroot/pkgsrc/doc/guide/files/fixes.xml,v
retrieving revision 1.147
diff -u -p -r1.147 fixes.xml
--- fixes.xml	2 Jan 2019 18:48:21 -0000	1.147
+++ fixes.xml	4 May 2019 12:54:12 -0000
@@ -291,7 +291,8 @@ ACCEPTABLE_LICENSES+=xv-license
     <para>Your package may depend on some other package being present
     - and there are various ways of expressing this dependency.
     pkgsrc supports the <varname>BUILD_DEPENDS</varname> and
-    <varname>DEPENDS</varname> definitions, the
+    <varname>DEPENDS</varname> and
+    <varname>TOOL_DEPENDS</varname> definitions, the
     <varname>USE_TOOLS</varname> definition, as well as dependencies
     via <filename>buildlink3.mk</filename>, which is the preferred way
     to handle dependencies, and which uses the variables named above.
@@ -301,15 +302,24 @@ ACCEPTABLE_LICENSES+=xv-license
     follows: The <varname>DEPENDS</varname> definition registers
     that pre-requisite in the binary package so it will be pulled in
     when the binary package is later installed, whilst the
-    <varname>BUILD_DEPENDS</varname> definition does not, marking a
-    dependency that is only needed for building the package.</para>
+    <varname>BUILD_DEPENDS</varname> and <varname>TOOL_DEPENDS</varname>
+    definitions does not, marking a dependency that is only needed for
+    building the package.</para>
 
     <para>This means that if you only need a package present whilst
     you are building, it should be noted as a
-    <varname>BUILD_DEPENDS</varname>.</para>
-
-    <para>The format for a <varname>BUILD_DEPENDS</varname> and a
-    <varname>DEPENDS</varname> definition is:</para>
+    <varname>TOOL_DEPENDS</varname> or
+    <varname>BUILD_DEPENDS</varname>.  When cross-compiling,
+    <varname>TOOL_DEPENDS</varname> are <emphasis>native</emphasis>
+    packages, i.e. packages for the architecture where the package
+    is built;
+    <varname>BUILD_DEPENDS</varname> are <emphasis>target</emphasis>
+    packages, i.e. packages for the architecture for which the package
+    is built.</para>
+
+    <para>The format for <varname>BUILD_DEPENDS</varname>,
+    <varname>DEPENDS</varname> and <varname>TOOL_DEPENDS</varname>
+    definition is:</para>
 
 <programlisting>
 &lt;pre-req-package-name&gt;:../../&lt;category&gt;/&lt;pre-req-package&gt;
@@ -346,10 +356,20 @@ ACCEPTABLE_LICENSES+=xv-license
 
       <listitem>
 	<para>If your package needs binaries from another package to build,
+	use the <varname>TOOL_DEPENDS</varname> definition:</para>
+
+<programlisting>
+TOOL_DEPENDS+= itstool-[0-9]*:../../textproc/itstool
+</programlisting>
+      </listitem>
+
+      <listitem>
+	<para>If your package needs static libraries to link against, header
+	files to include, etc. from another package to build,
 	use the <varname>BUILD_DEPENDS</varname> definition:</para>
 
 <programlisting>
-BUILD_DEPENDS+= itstool-[0-9]*:../../textproc/itstool
+BUILD_DEPENDS+= TODO: find a good example
 </programlisting>
       </listitem>
 
Index: tools.xml
===================================================================
RCS file: /cvsroot/pkgsrc/doc/guide/files/tools.xml,v
retrieving revision 1.7
diff -u -p -r1.7 tools.xml
--- tools.xml	11 Jun 2016 20:19:36 -0000	1.7
+++ tools.xml	4 May 2019 12:54:12 -0000
@@ -5,7 +5,7 @@
 
 <para>The <varname>USE_TOOLS</varname> definition is used both internally
 by pkgsrc and also for individual packages to define what commands
-are needed for building a package (like <varname>BUILD_DEPENDS</varname>)
+are needed for building a package (like <varname>TOOL_DEPENDS</varname>)
 or for later run-time of an installed packaged (such as
 <varname>DEPENDS</varname>).
 If the native system provides an adequate tool, then in many cases, a pkgsrc