Do not override PTLIBDIR in the environment.

Index: configure
--- configure.orig
+++ configure
@@ -4121,7 +4121,9 @@ $as_echo "yes" >&6; }
 	HAS_PTLIB=1
 fi
   if test ${HAS_PTLIB}x = 1x; then
-    PTLIBDIR=`${PKG_CONFIG} --variable=prefix ptlib`
+    if test "${PTLIBDIR:-unset}" = "unset" ; then
+      PTLIBDIR=`${PKG_CONFIG} --variable=prefix ptlib`
+    fi
     PTVERSION=`${PKG_CONFIG} --modversion ptlib`
     target_os=`${PKG_CONFIG} --variable=target_os ptlib`
     target_cpu=`${PKG_CONFIG} --variable=target_cpu ptlib`
