configure: do not test for CXX, it's not needed

There is no C++ source file, so no reason to require a C++ compiler.

Reported-by: Peter Korsgaard <jacmet@uclibc.org>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

---
Patch sent upstream
( not yet in archives, awaits moderator approval... :-/ )

diff -durN vde2-2.3.2.orig/configure.ac vde2-2.3.2/configure.ac
--- vde2-2.3.2.orig/configure.ac	2011-11-23 17:41:19.000000000 +0100
+++ vde2-2.3.2/configure.ac	2012-12-14 21:56:47.146319686 +0100
@@ -11,7 +11,6 @@
 AC_CONFIG_LIBOBJ_DIR(src/common)
 
 # Checks for programs.
-AC_PROG_CXX
 AC_PROG_CC
 AC_PROG_INSTALL
 AC_PROG_LN_S