Correctly check for g++ existence.

Copyright 2007 Yann E. MORIN <yann.morin.1998@anciens.enib.fr>
Licensed to you as dmalloc-5.5.2 is.

diff -dur dmalloc-5.5.2.orig/configure dmalloc-5.5.2/configure
--- dmalloc-5.5.2.orig/configure	2007-05-18 11:40:31.000000000 +0200
+++ dmalloc-5.5.2/configure	2007-05-18 11:42:02.000000000 +0200
@@ -2643,7 +2643,7 @@
 
 
 # see if we actually have a CXX program
-if test "$ac_cv_prog_CXX" = "" -o ! -x "$ac_cv_prog_CXX"; then
+if test "$ac_cv_prog_CXX" = "" -o ! -x `which "$ac_cv_prog_CXX"`; then
     { echo "$as_me:$LINENO: WARNING: could not find C++ compiler $ac_cv_prog_CXX" >&5
 echo "$as_me: WARNING: could not find C++ compiler $ac_cv_prog_CXX" >&2;}
     enable_cxx=no