configure on master checks for C++ compiler

[Eli Zaretskii]

I've noticed that the configure script now checks for a C++ compiler:

  checking for g++... g++
  checking whether we are using the GNU C++ compiler... yes
  checking whether g++ accepts -g... yes
  checking for g++ option to enable C++11 features... none needed
  checking how to run the C++ preprocessor... g++ -E

However, AFAIU the C++ compiler is really needed only for two builds:
Android and Haiku.  If that is correct, could we please limit these
tests only to those two builds, so that people who build for other
configurations won't need to have a C++ compiler installed?

Thanks.