Oops, exporting CXX for AC_CONFIG_SUBDIRS doesn't work if the user specified…

Oops, exporting CXX for AC_CONFIG_SUBDIRS doesn't work if the user specified their own CXX, in which case their version will be in ac_configure_args and will take precedence.  We'll have to modify ac_configure_args.

c++/configure.ac

@@ -26,15 +26,15 @@ AC_PROG_CXX
 AC_LANG([C++])
 AX_CXX_COMPILE_STDCXX_11([noext])
 ACX_PTHREAD
-
 AC_PROG_LIBTOOL
 
 LIBS="$PTHREAD_LIBS $LIBS"
 CXXFLAGS="$CXXFLAGS $PTHREAD_CFLAGS"
 
 # AX_CXX_COMPILE_STDCXX_11 may have modified CXX, and we need gtest to use these modifications as
-# well (especially -stdlib).
-export CXX
+# well (especially -stdlib).  Note that this may cause ac_configure_args to contain CXX= twice, but
+# the latter one should override the former.
+ac_configure_args="$ac_configure_args 'CXX=$CXX'"
 AC_CONFIG_SUBDIRS([gtest])
 
 AC_CONFIG_FILES([Makefile])