Fix usage of PTH_CONFIG to work with pthsem

The pthsem re-implement of pth provides a compatibility layer for pth,
but its pth-config script behaves slightly differently than the
original one when reporting the version number. This patch to gnupg2's
configure script adjusts the version checking to support this
difference, since Buildroot uses pth-config from pthsem.

We patch directly the configure script, because triggering the entire
autoreconf dance for just a one byte change in the configure script
seems a bit silly.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/configure
===================================================================
--- a/configure
+++ b/configure
@@ -8346,7 +8346,7 @@
   tmp=1.3.7
   if test "$PTH_CONFIG" != "no"; then
 
-    _pth_version=`$PTH_CONFIG --version | awk 'NR==1 {print $3}'`
+    _pth_version=`$PTH_CONFIG --version | awk 'NR==1 {print $2}'`
     _req_version="$tmp"
 
     { $as_echo "$as_me:${as_lineno-$LINENO}: checking for PTH - version >= $_req_version" >&5