test fails when no C++ compiler is available

[Jim Meyering]

Hi Ralf,

I've just built automake-from-git on a newly-installed system
that lacked a C++ compiler.  It failed like this:

220: Multiple languages                              FAILED (compile.at:249)
...



220. compile.at:204: 220. Multiple languages (compile.at:204): FAILED 
(compile.at:249)

tests/testsuite.dir/220/config.log:

  configure:2973: result: no
  configure:3003: checking a simple C program that is not valid C++
  configure:3018: gcc -c -g -O2  conftest.c >&5
  configure:3018: $? = 0
  configure:3019: result: ok
  configure:3040: checking a simple C++ program that is not valid C
  configure:3054: g++ -c   conftest.cpp >&5
  ./configure: line 1401: g++: command not found
  configure:3054: $? = 127
  configure: failed program was:
  | /* confdefs.h */
  | #define PACKAGE_NAME ""
  | #define PACKAGE_TARNAME ""
  | #define PACKAGE_VERSION ""
  | #define PACKAGE_STRING ""
  | #define PACKAGE_BUGREPORT ""
  | #define PACKAGE_URL ""
  | /* end confdefs.h.  */
  | class A {};
  | int
  | main ()
  | {
  |
  |   ;
  |   return 0;
  | }
  configure:3058: result: failed
  configure:3060: error: could not compile test program