dpkg-buildpackage(1): drop mention of PKG_CONFIG_LIBDIR

The feature has been removed in dpkg 1.15.6.

debian/changelog

@@ -21,6 +21,8 @@ dpkg (1.15.7.2) UNRELEASED; urgency=low
     Closes: #575410
   * Update reference to triggers.txt.gz in dpkg-trigger(1) and deb-triggers(5)
     to match the new location. Closes: #580774
+  * Drop mention of PKG_CONFIG_LIBDIR in dpkg-buildpackage(1), the feature has
+    been removed in 1.15.6.
 
   [ Guillem Jover ]
   * Add powerpcspe support to ostable and triplettable.

man/dpkg-buildpackage.1

@@ -102,12 +102,6 @@ Passed unchanged to \fBdpkg-genchanges\fP. See its manual page.
 Specify the Debian architecture we build for. The architecture of the
 machine we build on is determined automatically, and is also the default
 for the host machine.
-
-If the host architecture differs from the build architecture (as is the
-case for a cross-compilation), and if the environment variable
-\fBPKG_CONFIG_LIBDIR\fP is not set, then it is set to a value suitable for
-cross-compilation
-("/usr/\fIgnu-system-type\fP/lib/pkgconfig/:/usr/share/pkgconfig").
 .TP
 .BI \-t gnu-system-type
 Specify the GNU system type we build for. It can be used in place