check in configure / make that cc,gcc, and g++ point to same compiler

[cartazio]

see discussion in #10941

basically should look at the respective outputs of cc,gcc,g++ on the flag -v or --version and check that they agree

[cartazio]

this issue is most likely to come up for developers on macs, because xcode now ships with gcc/g++ pointing to clang, which creates funny build problems, so some devs on mac need to install a real gcc and have it in their path

[cartazio]

i'll see if I can figure out enough sh/bash to do a candidate PR for this, nb that i'm pretty weak at shell and I don't have that much hobby hacking currently

[huonw]

Closed by #10964.